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  • 1.
    Andersson, Jonas
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Kaiser, Franz-Joachim
    Gómez, Faustino
    Jäkel, Oliver
    Pardo-Montero, Juan
    Tölli, Heikki
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    A comparison of different experimental methods for general recombination correction for liquid ionization chambers2012In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 21, p. 7161-7175Article in journal (Refereed)
    Abstract [en]

    Radiation dosimetry of highly modulated dose distributions requires a detector with a high spatial resolution. Liquid filled ionization chambers (LICs) have the potential to become a valuable tool for the characterization of such radiation fields. However, the effect of an increased recombination of the charge carriers, as compared to using air as the sensitive medium has to be corrected for. Due to the presence of initial recombination in LICs, the correction for general recombination losses is more complicated than for air-filled ionization chambers. In the present work, recently published experimental methods for general recombination correction for LICs are compared and investigated for both pulsed and continuous beams. The experimental methods are all based on one of two approaches, either measurements at two different dose rates (two-dose-rate methods), or measurements at three different LIC polarizing voltages (three-voltage methods). In a comparison with the two-dose-rate methods, the three-voltage methods fail to achieve accurate corrections in several instances, predominantly at low polarizing voltages and dose rates. However, for continuous beams in the range of polarizing voltages recommended by the manufacturer of the LICs used, the agreement between the different methods is generally within the experimental uncertainties. For pulsed beams, the agreement between the methods is poor. The inaccuracies found in the results from the three-voltage methods are associated with numerical difficulties in solving the resulting equation systems, which also make these methods sensitive to small variations in the experimental data. These issues are more pronounced for the case of pulsed beams. Furthermore, the results suggest that the theoretical modelling of initial recombination used in the three-voltage methods may be a contributing factor to the deviating results observed.

  • 2.
    Andersson, Jonas
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Tölli, Heikki
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams2011In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 56, no 2, p. 299-314Article in journal (Refereed)
    Abstract [en]

    A method to correct for the general recombination losses for liquid ionization chambers in continuous beams has been developed. The proposed method has been derived from Greening's theory for continuous beams and is based on measuring the signal from a liquid ionization chamber and an air filled monitor ionization chamber at two different dose rates. The method has been tested with two plane parallel liquid ionization chambers in a continuous radiation x-ray beam with a tube voltage of 120 kV and with dose rates between 2 and 13 Gy min-1. The liquids used as sensitive media in the chambers were isooctane (C8H18) and tetramethylsilane (Si(CH3)4). The general recombination effect was studied using chamber polarizing voltages of 100, 300, 500, 700 and 900 V for both liquids. The relative standard deviation of the results for the collection efficiency with respect to general recombination was found to be a maximum of 0.7 % for isooctance and 2.4 % for tetramethylsilane. The results are in excellent agreement with Greening's theory for collection efficiencies over 90 %. The measured and corrected signals from the liquid ionization chambers used in this work are in very good agreement with the air filled monitor chambers with respect to signal to dose linearity.

  • 3. Andersson, Martin
    et al.
    Mattsson, Soren
    Johansson, Lennart
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Leide-Svegborn, Sigrid
    A biokinetic and dosimetric model for ionic indium in humans2017In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 62, no 16, p. 6397-6407Article in journal (Refereed)
    Abstract [en]

    This paper reviews biokinetic data for ionic indium, and proposes a biokinetic model for systemic indium in adult humans. The development of parameter values focuses on human data and indium in the form of ionic indium(III), as indium chloride and indium arsenide. The model presented for systemic indium is defined by five different pools: plasma, bone marrow, liver, kidneys and other soft tissues. The model is based on two subsystems: one corresponding to indium bound to transferrin and one where indium is transported back to the plasma, binds to red blood cell transferrin and is then excreted through the kidneys to the urinary bladder. Absorbed doses to several organs and the effective dose are calculated for In-111- and In-113m-ions. The proposed biokinetic model is compared with previously published biokinetic indium models published by the ICRP. The absorbed doses are calculated using the ICRP/ICRU adult reference phantoms and the effective dose is estimated according to ICRP Publication 103. The effective doses for In-111 and In-113m are 0.25 mSv MBq(-1) and 0.013 mSv MBq(-1) respectively. The updated biokinetic and dosimetric models presented in this paper take into account human data and new animal data, which represent more detailed and presumably more accurate dosimetric data than that underlying previous models for indium.

  • 4.
    Andersson, Martin
    et al.
    Medical Radiation Physics, Department of Clinical Sciences Malmö, Lund University, Skåne University Hospital, Malmö, Sweden .
    Minarik, David
    Medical Radiation Physics, Department of Clinical Sciences Malmö, Lund University, Skåne University Hospital, Malmö, Sweden .
    Johansson, Lennart
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Mattsson, Soren
    Medical Radiation Physics, Department of Clinical Sciences Malmö, Lund University, Skåne University Hospital, Malmö, Sweden .
    Leide-Svegborn, Sigrid
    Medical Radiation Physics, Department of Clinical Sciences Malmö, Lund University, Skåne University Hospital, Malmö, Sweden .
    Improved estimates of the radiation absorbed dose to the urinary bladder wall2014In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 59, no 9, p. 2173-2182Article in journal (Refereed)
    Abstract [en]

    Specific absorbed fractions (SAFs) have been calculated as a function of the content in the urinary bladder in order to allow more realistic calculations of the absorbed dose to the bladder wall. The SAFs were calculated using the urinary bladder anatomy from the ICRP male and female adult reference computational phantoms. The urinary bladder and its content were approximated by a sphere with a wall of constant mass, where the thickness of the wall depended on the amount of urine in the bladder. SAFs were calculated for males and females with 17 different urinary bladder volumes from 10 to 800 mL, using the Monte Carlo computer program MCNP5, at 25 energies of mono-energetic photons and electrons ranging from 10 KeV to 10 MeV. The decay was assumed to be homogeneously distributed in the urinary bladder content and the urinary bladder wall, and the mean absorbed dose to the urinary bladder wall was calculated. The Monte Carlo simulations were validated against measurements made with thermoluminescent dosimeters. The SAFs obtained for a urine volume of 200 mL were compared to the values calculated for the urinary bladder wall using the adult reference computational phantoms. The mean absorbed dose to the urinary wall from F-18-FDG was found to be 77 mu Gy/MBq formales and 86 mu Gy/MBq for females, while for (99)mTc-DTPA the mean absorbed doses were 80 mu Gy/MBq for males and 86 mu Gy/MBq for females. Compared to calculations using a constant value of the SAF from the adult reference computational phantoms, the mean absorbed doses to the bladder wall were 60% higher for F-18-FDG and 30% higher for (99)mTc-DTPA using the new SAFs.

  • 5.
    Blomquist, M
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, M G
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Zackrisson, B
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Multileaf collimation of electrons-clinical effects on electron energy modulation and mixed beam therapy depending on treatment head design2002In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 47, no 7, p. 1013-1024Article in journal (Refereed)
    Abstract [en]

    The aim Of this Study was to explore the possibilities of using multileaf-collimated electron beams for advanced radiation therapy with conventional scattering foil flattened beams. Monte Carlo simulations were performed with the aim to improve electron beam characteristics and enable isocentric multileaf collimation. The scattering foil positions, monitor chamber thickness, the MLC location and the amount of He in the treatment head were optimized for three common commercial accelerators. The performance of the three optimized treatment head designs was compared for different SSDs in air. at treatment depth in water and for some clinical cases. The effects of electron/photon beam matching including generalized random and static errors using Gaussian one-dimensional (1D) error distributions, and also electron energy modulation, were studied at treatment depth in beater, The modification of the treatment heads improved the electron beam characteristics and enabled the use of multileaf collimation in isocentric delivery of both electron and photon beams in a mixed beam IMRT procedure.

  • 6.
    Blomquist, M
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Measured lung dose correction factors for 50 MV photons1998In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 43, no 11, p. 3225-3234Article in journal (Refereed)
    Abstract [en]

    Some clinically relevant measurements of lung tissue/water equivalent interfaces have been performed for a 50 MV therapeutic x-ray beam. The purpose was to investigate the severity of dose perturbation effects in lung tissue and adjacent tissues using an energy well above the common clinical practice in thoracic irradiations. The phantoms were constructed of solid water, PMMA and white polystyrene as soft tissue (water) equivalents, and cork was used as the lung tissue equivalent. Measurements were performed using radiographic film and a cylindrical ionization chamber. The results show that the degradation of the 20/80% beam penumbra in the lung region is severe, up to 2.5 times the penumbra in water for a 10 cm thick lung with a density of 0.30 x 10(3) kg m(-3). The lack of electronic equilibrium in the low-density region can cause underdosage at the lung/tumour interface of up to 30% of maximum target dose, and the build-up depth to 95% of target dose in unit density tissue behind the lung may be as large as 22 mm. It is also shown that these figures strongly depend on patient anatomy and beam size and why a careful calculation of the individual dose distribution is needed for optimal choice of photon beam energy in thoracic treatments.

  • 7.
    Blomquist, M
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Ma, C M
    Zackrisson, B
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
    Comments on 'X-ray energy choice for lung tumour irradiation depends on the density distribution of clonogenic cells'2003In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 48, no 8, p. L29-L30Article in journal (Refereed)
  • 8.
    Blomquist, M
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Li, J S
    Ma, C M
    Zackrisson, B
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Comparison between a conventional treatment energy and 50 MV photons for the treatment of lung tumours2002In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 47, no 6, p. 889-897Article in journal (Refereed)
    Abstract [en]

    Radiation therapy in the thoracic region is difficult due to the presence of many dose-limiting structures and the large density differences that affect the dose distribution. Conventional irradiation techniques use low-energy photon beams to avoid build-up effects superficially in the tumour and increased lateral scattering of the beams. For deep-seated tumours higher beam energies could have lung-sparing properties that would enable dose escalation. A comparison was made for a conventional low photon energy (6 MV) and 50 MV photons for the treatment of a lung tumour. A representative patient geometry was selected, consisting of a small tumour semi-enclosed in lung tissue. Treatment plans were designed using a commercial 3D-pencil beam treatment planning system. The treatment beams designed in the TPS were simulated with the Monte Carlo code EGS4/BEAM and the dose distribution in the phantom created from the patients CT-data was calculated using MCDOSE with identical beam geometry for both energies. The intrinsic difference between the two photon energies implies a sparing effect of lung that can be utilized for dose escalation. For a treatment with two beams the mean total dose to the tumour could be increased by 5.3% for 50 MV, corresponding to 3.2 Gy for a prescription dose of 60 Gy, with the same complication probability for the treated lung as for 6 MV. In conclusion, high-energy beams have qualities that can be taken advantage of for irradiation of lung tumours. Optimum solutions would probably require the use of both high- and low-energy beams.

  • 9.
    Blomquist, M
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Satherberg, A
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Zackrisson, B
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
    Scanned intensity modulations for 50 MV photons1998In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 43, no 5, p. 1185-1197Article in journal (Refereed)
    Abstract [en]

    Optimization of the dose distributions by individual beam compensation is a useful tool in conformal radiation therapy. Intensity modulation by electromagnetic scanning of a narrow elementary beam allows fast dose delivery and causes little change in beam quality compared with other methods, especially for high energies such as 50 MV. Intensity modulated beams from the MM50 accelerator were measured and compared with calculations based on Monte Carlo simulations. Good agreement between measurements and calculations were found, typically within 1% for central dose profiles. The steepest wedge angle that was produced with the scanning beam technique was of 45 degrees or 3.5% cm(-1) for a 20 cm x 20 cm field, slightly varying with depth. The elementary 50 MV photon 'pencil beam' for a full range, high-z bremsstrahlung target, is a wide dose distribution at 10 cm depth in water which limits the modulation gradient and hence the complexity of the modulation by the scanning of a photon pencil beam only. Scanned wedge beam distributions were modelled in the treatment planning system and a pelvic treatment with three fields was used to illustrate a clinical application. The resulting dose volume data were compared for different radiation qualities but with similar beam portals. 'Energy modulation' by field matching with lower photon energies was performed to sharpen the penumbra towards organs at risk.

  • 10.
    Dasu, Alexandru
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Toma-Dasu, Iuliana
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Theoretical simulation of tumour oxygenation and results from acute and chronic hypoxia2003In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 48, no 17, p. 2829-2842Article in journal (Refereed)
    Abstract [en]

    The tumour microenvironment is considered to be responsible for the outcome of cancer treatment and therefore it is extremely important to characterize and quantify it. Unfortunately, most of the experimental techniques available now are invasive and generally it is not known how this influences the results. Non-invasive methods on the other hand have a geometrical resolution that is not always suited for the modelling of the tumour response. Theoretical simulation of the microenvironment may be an alternative method that can provide quantitative data for accurately describing tumour tissues.

    This paper presents a computerized model that allows the simulation of the tumour oxygenation. The model simulates numerically the fundamental physical processes of oxygen diffusion and consumption in a two-dimensional geometry in order to study the influence of the different parameters describing the tissue geometry. The paper also presents a novel method to simulate the effects of diffusion-limited (chronic) hypoxia and perfusion-limited (acute) hypoxia.

    The results show that all the parameters describing tissue vasculature are important for describing tissue oxygenation. Assuming that vascular structure is described by a distribution of inter-vessel distances, both the average and the width of the distribution are needed in order to fully characterize the tissue oxygenation. Incomplete data, such as distributions measured in a non-representative region of the tissue, may not give relevant tissue oxygenation.

    Theoretical modelling of tumour oxygenation also allows the separation between acutely and chronically hypoxic cells, a distinction that cannot always be seen with other methods. It was observed that the fraction of acutely hypoxic cells depends not only on the fraction of collapsed blood vessels at any particular moment, but also on the distribution of vessels in space as well.

    All these suggest that theoretical modelling of tissue oxygenation starting from the basic principles is a robust method that can be used to quantify the tissue oxygenation and to provide input parameters for other simulations.

  • 11.
    Daşu, Alexandru
    et al.
    Umeå University, Faculty of Medicine, Radiation Sciences.
    Fowler, Jack F
    Comments on "Comparison of in vitro and in vivo alpha/beta ratios for prostate cancer".2005In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 50, no 6, p. L1-4; author reply L5Article in journal (Refereed)
  • 12. Georg, Dietmar
    et al.
    Olofsson, Jörgen
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Künzler, Thomas
    Aiginger, Hannes
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    A practical method to calculate head scatter factors in wedged rectangular and irregular MLC shaped beams for external and internal wedges2004In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 49, no 20, p. 4689-4700Article in journal (Refereed)
    Abstract [en]

    Factor based methods for absorbed dose or monitor unit calculations are often based on separate data sets for open and wedged beams. The determination of basic beam parameters can be rather time consuming, unless equivalent square methods are applied. When considering irregular wedged beams shaped with a multileaf collimator, parametrization methods for dosimetric quantities, e.g. output ratios or wedge factors as a function of field size and shape, become even more important. A practical method is presented to derive wedged output ratios in air (S-c,S-w) for any rectangular field and for any irregular MLC shaped beam. This method was based on open field output ratios in air (Sc) for a field with the same collimator setting, and a relation f(w) between S-c,S-w and S-c. The relation f(w) can be determined from measured output ratios in air for a few open and wedged fields including the maximum wedged field size. The function fw and its parametrization were dependent on wedge angle and treatment head design, i.e. they were different for internal and external wedges. The proposed method was tested for rectangular wedged fields on three accelerators with internal wedges (GE, Elekta, BBC) and two accelerators with external wedges (Varian). For symmetric regular beams the average deviation between calculated and measured S-c,S-w/S-c ratios was 0.3% for external wedges and about 0.6% for internal wedges. Maximum deviations of 1.8% were obtained for elongated rectangular fields on the GE and ELEKTA linacs with an internal wedge. The same accuracy was achieved for irregular MLC shaped wedged beams on the accelerators with MLC and internal wedges (GE and Elekta), with an average deviation < 1 % for the fields tested. The proposed method to determine output ratios in air for wedged beams from output ratios of open beams, combined with equivalent square approaches, can be easily integrated in empirical or semi-empirical methods for monitor unit calculations. 

  • 13. Georg, Dietmar
    et al.
    Stock, Markus
    Kroupa, Bernhard
    Olofsson, Jörgen
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Nyholm, Tufve
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Ahnesjö, Anders
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Patient-specific IMRT verification using independent fluence-based dose calculation software: experimental benchmarking and initial clinical experience2007In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 52, no 16, p. 4981-4992Article in journal (Refereed)
    Abstract [en]

    Experimental methods are commonly used for patient-specific intensity-modulated radiotherapy (IMRT) verification. The purpose of this study was to investigate the accuracy and performance of independent dose calculation software ( denoted as 'MUV' ( monitor unit verification)) for patient-specific quality assurance (QA). 52 patients receiving step-and-shoot IMRT were considered. IMRT plans were recalculated by the treatment planning systems (TPS) in a dedicated QA phantom, in which an experimental 1D and 2D verification (0.3 cm(3) ionization chamber; films) was performed. Additionally, an independent dose calculation was performed. The fluence-based algorithm of MUV accounts for collimator transmission, rounded leaf ends, tongue-and-groove effect, backscatter to the monitor chamber and scatter from the flattening filter. The dose calculation utilizes a pencil beam model based on a beam quality index. DICOM RT files from patient plans, exported from the TPS, were directly used as patient-specific input data in MUV. For composite IMRT plans, average deviations in the high dose region between ionization chamber measurements and point dose calculations performed with the TPS and MUV were 1.6 +/- 1.2% and 0.5 +/- 1.1% ( 1 S. D.). The dose deviations between MUV and TPS slightly depended on the distance from the isocentre position. For individual intensity-modulated beams ( total 367), an average deviation of 1.1 +/- 2.9% was determined between calculations performed with the TPS and with MUV, with maximum deviations up to 14%. However, absolute dose deviations were mostly less than 3 cGy. Based on the current results, we aim to apply a confidence limit of 3% ( with respect to the prescribed dose) or 6 cGy for routine IMRT verification. For off-axis points at distances larger than 5 cm and for low dose regions, we consider 5% dose deviation or 10 cGy acceptable. The time needed for an independent calculation compares very favourably with the net time for an experimental approach. The physical effects modelled in the dose calculation software MUV allow accurate dose calculations in individual verification points. Independent calculations may be used to replace experimental dose verification once the IMRT programme is mature.

  • 14.
    Johansson, Erik
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Andersson, Jonas
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Johansson, Lennart
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Tölli, Heikki
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Liquid ionization chamber initial recombination dependence on LET for electrons and photons2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 12, p. 4225-4236Article in journal (Refereed)
    Abstract [en]

    The possibility of indirect measurements of linear energy transfer (LET) with a liquid ionization chamber (LIC) has been investigated by studying initial recombination losses at different applied voltages. A linear fit is made to the voltage-signal curve and the intersection point of the fit and the voltage-axis is shown to correlate with LET. The LIC applied voltages were 100-700 V, which corresponds to electric field strengths between 0.3 and 2.0 MV m(-1). Several different photon and electron beams have been studied, and by using MCNPX (TM) the respective LET spectra have been determined. The beam qualities in this study were found to have a fluence averaged LET between 0.17 and 1.67 keV mu m(-1) and a corresponding dose averaged LET between 0.97 and 4.62 keV mu m(-1). For the experimental data in this study the linear fit method yields consistent results with respect to Monte Carlo simulated LET values. A calibration curve for LET determination is provided for the LIC used in the present work.

  • 15.
    Jonasson, Lars
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Diagnostic Radiology. Umeå University, Faculty of Medicine, Umeå Centre for Functional Brain Imaging (UFBI). Umeå University, Faculty of Social Sciences, Centre for Demographic and Ageing Research (CEDAR).
    Axelsson, Jan
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics. Umeå University, Faculty of Medicine, Umeå Centre for Functional Brain Imaging (UFBI).
    Riklund, Katrine
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Diagnostic Radiology. Umeå University, Faculty of Medicine, Umeå Centre for Functional Brain Imaging (UFBI).
    Boraxbekk, Carl-Johan
    Umeå University, Faculty of Medicine, Umeå Centre for Functional Brain Imaging (UFBI). Umeå University, Faculty of Social Sciences, Centre for Demographic and Ageing Research (CEDAR). Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital, Hvidovre, Denmark.
    Simulating effects of brain atrophy in longitudinal PET imaging with an anthropomorphic brain phantom2017In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 62, no 13, p. 5213-5227Article in journal (Refereed)
    Abstract [en]

    In longitudinal positron emission tomography (PET), the presence of volumetric changes over time can lead to an overestimation or underestimation of the true changes in the quantified PET signal due to the partial volume effect (PVE) introduced by the limited spatial resolution of existing PET cameras and reconstruction algorithms. Here, a 3D-printed anthropomorphic brain phantom with attachable striata in three sizes was designed to enable controlled volumetric changes. Using a method to eliminate the non-radioactive plastic wall, and manipulating BP levels by adding different number of events from list-mode acquisitions, we investigated the artificial volume dependence of BP due to PVE, and potential bias arising from varying BP. Comparing multiple reconstruction algorithms we found that a high-resolution ordered- subsets maximization algorithm with spatially variant point-spread function resolution modeling provided the most accurate data. For striatum, the BP changed by 0.08% for every 1% volume change, but for smaller volumes such as the posterior caudate the artificial change in BP was as high as 0.7% per 1% volume change. A simple gross correction for striatal volume is unsatisfactory, as the amplitude of the PVE on the BP differs depending on where in the striatum the change occurred. Therefore, to correctly interpret age-related longitudinal changes in the BP, we must account for volumetric changes also within a structure, rather than across the whole volume. The present 3D-printing technology, combined with the wall removal method, can be implemented to gain knowledge about the predictable bias introduced by the PVE differences in uptake regions of varying shape.

  • 16.
    Kallioniemi, Antti
    et al.
    Department of Physics, University of Kuopio, Kuopio, Finland.
    Jurvelin, Jukka
    Department of Physics, University of Kuopio, Kuopio, Finland; Department of Clinical Physiology and Nuclear Medicine, Kuopio University Hospital, Kuopio, Finland.
    Nieminen, Miika
    Department of Diagnostic Radiology, Oulu University Hospital, Oulu, Finland .
    Lammi, Mikko
    Department of Anatomy, Institute of Biomedicine, University of Kuopio, Kuopio, Finland .
    Töyräs, Juha
    Department of Clinical Neurophysiology, Kuopio University Hospital, Kuopio, Finland.
    Contrast agent enhanced pQCT of articular cartilage.2007In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 52, no 4, p. 1209-1219, article id 17264381Article in journal (Refereed)
    Abstract [en]

    The delayed gadolinium enhanced MRI of cartilage (dGEMRIC) technique is the only non-invasive means to estimate proteoglycan (PG) content in articular cartilage. In dGEMRIC, the anionic paramagnetic contrast agent gadopentetate distributes in inverse relation to negatively charged PGs, leading to a linear relation between T1,Gd and spatial PG content in tissue. In the present study, for the first time, contrast agent enhanced peripheral quantitative computed tomography (pQCT) was applied, analogously to dGEMRIC, for the quantitative detection of spatial PG content in cartilage. The suitability of two anionic radiographic contrast agents, gadopentetate and ioxaglate, to detect enzymatically induced PG depletion in articular cartilage was investigated. First, the interrelationships of x-ray absorption, as measured with pQCT, and the contrast agent solution concentration were investigated. Optimal contrast agent concentrations for the following experiments were selected. Second, diffusion rates for both contrast agents were investigated in intact (n=3) and trypsin-degraded (n=3) bovine patellar cartilage. The contrast agent concentration of the cartilaginous layer was measured prior to and 2-27 h after immersion. Optimal immersion time for the further experiments was selected. Third, the suitability of gadopentetate and ioxaglate enhanced pQCT to detect the enzymatically induced specific PG depletion was investigated by determining the contrast agent concentrations and uronic acid and water contents in digested and intact osteochondral samples (n=16). After trypsin-induced PG loss (-70%, p<0.05) the penetration of gadopentetate and ioxaglate increased (p<0.05) by 34% and 48%, respectively. Gadopentetate and ioxaglate concentrations both showed strong correlation (r=-0.95, r=-0.94, p<0.01, respectively) with the uronic acid content. To conclude, contrast agent enhanced pQCT provides a technique to quantify PG content in normal and experimentally degraded articular cartilage in vitro. As high resolution imaging of e.g. the knee joint is possible with pQCT, the present technique may be further developed for in vivo quantification of PG depletion in osteoarthritic cartilage. However, careful in vitro and in vivo characterization of diffusion mechanics and optimal contrast agent concentrations are needed before diagnostic applications are feasible.

  • 17. Kuess, Peter
    et al.
    Andrzejewski, Piotr
    Nilsson, David
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Georg, Petra
    Knoth, Johannes
    Susani, Martin
    Trygg, Johan
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Helbich, Thomas H.
    Polanec, Stephan H.
    Georg, Dietmar
    Nyholm, Tufve
    Umeå University, Faculty of Medicine, Department of Radiation Sciences. Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden.
    Association between pathology and texture features of multi parametric MRI of the prostate2017In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 62, no 19, p. 7833-7854Article in journal (Refereed)
    Abstract [en]

    The role of multi-parametric (mp)MRI in the diagnosis and treatment of prostate cancer has increased considerably. An alternative to visual inspection of mpMRI is the evaluation using histogram-based (first order statistics) parameters and textural features (second order statistics). The aims of the present work were to investigate the relationship between benign and malignant sub-volumes of the prostate and textures obtained from mpMR images. The performance of tumor prediction was investigated based on the combination of histogram-based and textural parameters. Subsequently, the relative importance of mpMR images was assessed and the benefit of additional imaging analyzed. Finally, sub-structures based on the PI-RADS classification were investigated as potential regions to automatically detect maligned lesions. Twenty-five patients who received mpMRI prior to radical prostatectomy were included in the study. The imaging protocol included T2, DWI, and DCE. Delineation of tumor regions was performed based on pathological information. First and second order statistics were derived from each structure and for all image modalities. The resulting data were processed with multivariate analysis, using PCA (principal component analysis) and OPLS-DA (orthogonal partial least squares discriminant analysis) for separation of malignant and healthy tissue. PCA showed a clear difference between tumor and healthy regions in the peripheral zone for all investigated images. The predictive ability of the OPLS-DA models increased for all image modalities when first and second order statistics were combined. The predictive value reached a plateau after adding ADC and T2, and did not increase further with the addition of other image information. The present study indicates a distinct difference in the signatures between malign and benign prostate tissue. This is an absolute prerequisite for automatic tumor segmentation, but only the first step in that direction. For the specific identified signature, DCE did not add complementary information to T2 and ADC maps.

  • 18. Laitano, RF
    et al.
    Guerra, AS
    Pimpinella, M
    Nyström, H
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Svensson, Hans
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Correction factors for calibration of plane-parallel ionization chambers with a Co-60 gamma-ray beam1993In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 38, no 1, p. 39-54Article in journal (Refereed)
    Abstract [en]

    The appropriate correction factors have been determined to enable plane-parallel ionization chambers to be calibrated using a Co-60 photon beam with a known air kerma rate. These factors refer to different calibration conditions and to different types of plane-parallel chambers. The main purpose of these correction factors is to allow calibration procedures more widely practicable than those based on the use of an electron beam of sufficiently high energy. The condition required to apply the correction factors is that the characteristics of the chambers to be calibrated are the same as those reported for the chambers considered in this investigation. To this end the types of chambers investigated were among the most widely used plane-parallel chambers commercially available. The results of this work also give indications on the properties of the plane-parallel chambers with regard to the chamber-phantom matching.

  • 19.
    Larsson, Anne
    et al.
    Umeå University, Faculty of Medicine, Radiation Sciences, Radiation Physics.
    Jakobson Mo, Susanna
    Umeå University, Faculty of Medicine, Radiation Sciences, Diagnostic Radiology.
    Ljungberg, Michael
    Riklund, Katrine
    Umeå University, Faculty of Medicine, Radiation Sciences, Diagnostic Radiology.
    Dopamine D2 receptor SPECT with (123)I-IBZM: evaluation of collimator and post-filtering when using model-based compensation-a Monte Carlo study.2010In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 7, p. 1971-1988Article in journal (Refereed)
    Abstract [en]

    In (123)I-IBZM brain SPECT, the main interest is the activity uptake in the striatum relative to the background, and semi-quantitative techniques using regions of interest are typically used for this purpose. Uncertainties in the measured uptakes can however be a problem due to low contrasts and high noise levels. Like SPECT in general, IBZM SPECT should benefit from reconstruction methods that include model-based compensation, but it is important that image acquisition is optimized for this technique. An important factor is the choice of collimator. In this study we compare four different parallel-hole collimators for IBZM SPECT regarding overall quantitative accuracy and measured uptake ratio as a function of image noise and uncertainty. The collimators are low-energy high-resolution (LEHR), low-energy general-purpose (LEGP), extended LEGP (ELEGP) and medium-energy general-purpose (MEGP). The effect of three Butterworth post-filters with cut-off frequencies of 0.3, 0.45 and 0.6 cm(-1) (power factor 8) is also studied. All raw-data projections are produced using Monte Carlo simulations. Of the investigated collimators, the one that is most sensitive to the primary photons, ELEGP, proved to be the most optimal for realistic noise levels. Butterworth post-filtering is advantageous, and the cut-off frequency 0.45 cm(-1) was the best compromise in this study.

  • 20.
    Larsson, Anne
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Johansson, Lennart
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Scatter-to-primary based scatter fractions for transmission dependent convolution subtraction of SPECT images2003In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 48, no 22, p. N323-N328Article in journal (Refereed)
    Abstract [en]

    In single photon emission computed tomography (SPECT), transmission-dependent convolution subtraction has been shown to be useful when correcting for scattered events. The method is based on convolution subtraction, but includes a matrix of scatter fractions instead of a global scatter fraction. The method can be extended to iteratively improve the scatter estimate, but in this note we show that this requires a modification of the theory to use scatter-to-total scatter fractions for the first iteration only and scatter-to-primary fractions thereafter. To demonstrate this, scatter correction is performed on a Monte Carlo simulated image of a point source of activity in water. The modification of the theory is compared to corrections where the scatter fractions are based on the scatter-to-total ratio, using one and ten iterations. The resulting ratios of subtracted to original counts are compared to the true scatter-to-total ratio of the simulation and the most accurate result is found for our modification of the theory.

  • 21.
    Mavroidis, Panayiotis
    et al.
    Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, Sweden .
    Lind, Bengt K
    Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, Sweden .
    Theodorou, Kyriaki
    Department of Medical Physics, Larissa University Hospital, Larissa, Greece .
    Laurell, Göran
    Department of Otolaryngology, Head and Neck Surgery, Karolinska Hospital, Stockholm, Sweden .
    Fernberg, Jan-Olof
    Department of Oncology, Radiumhemmet, Karolinska Hospital, Stockholm, Sweden .
    Lefkopoulos, Dimitrios
    Department of Radiation Physics, Tenon Hospital, Paris, France .
    Kappas, Constantin
    Department of Medical Physics, Larissa University Hospital, Larissa, Greece .
    Brahme, Anders
    Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, Sweden .
    Statistical methods for clinical verification of dose-response parameters related to esophageal stricture and AVM obliteration from radiotherapy2004In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 49, no 16, p. 3797-3816Article in journal (Refereed)
    Abstract [en]

    The purpose of this work is to provide some statistical methods for evaluating the predictive strength of radiobiological models and the validity of dose-response parameters for tumour control and normal tissue complications. This is accomplished by associating the expected complication rates, which are calculated using different models, with the clinical follow-up records. These methods are applied to 77 patients who received radiation treatment for head and neck cancer and 85 patients who were treated for arteriovenous malformation (AVM). The three-dimensional dose distribution delivered to esophagus and AVM nidus and the clinical follow-up results were available for each patient. Dose-response parameters derived by a maximum likelihood fitting were used as a reference to evaluate their compatibility with the examined treatment methodologies. The impact of the parameter uncertainties on the dose-response curves is demonstrated. The clinical utilization of the radiobiological parameters is illustrated. The radiobiological models (relative seriality and linear Poisson) and the reference parameters are validated to prove their suitability in reproducing the treatment outcome pattern of the patient material studied (through the probability of finding a worse fit, area under the ROC curve and chi2 test). The analysis was carried out for the upper 5 cm of the esophagus (proximal esophagus) where all the strictures are formed, and the total volume of AVM. The estimated confidence intervals of the dose-response curves appear to have a significant supporting role on their clinical implementation and use.

  • 22. Nowik, Patrik
    et al.
    Bujila, Robert
    Kull, Love
    Andersson, Jonas
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Poludniowski, Gavin
    The dosimetric impact of including the patient table in CT dose estimates2017In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 62, no 23, p. 538-547Article in journal (Refereed)
    Abstract [en]

    The purpose of this study was to evaluate the dosimetric impact of including the patient table in Monte Carlo CT dose estimates for both spiral scans and scan projection radiographs (SPR). CT scan acquisitions were simulated for a Siemens SOMATOM Force scanner (Siemens Healthineers, Forchheim, Germany) with and without a patient table present. An adult male, an adult female and a pediatric female voxelized phantom were simulated. The simulated scans included tube voltages of 80 and 120 kVp. Spiral scans simulated without a patient table resulted in effective doses that were overestimated by approximately 5 % compared to the same simulations performed with the patient table present. Doses in selected individual organs (breast, colon, lung, red bone marrow and stomach) were overestimated by up to 8 %. Effective doses from SPR acquired with the X-ray tube stationary at 6 o'clock (posterior-anterior) were overestimated by 14-23 % when the patient table was not included, with individual organ dose discrepancies (breast, colon, lung red bone marrow and stomach) all exceeding 13%. The reference entrance skin dose to the back were in this situation overestimated by 6-15 %. These results highlight the importance of including the patient table in patient dose estimates for such scan situations.

  • 23.
    Nyholm, Tufve
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Olofsson, Jörgen
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Ahnesjö, Anders
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Georg, Dietmar
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Pencil kernel correction and residual error estimation for quality-index-based dose calculations2006In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 51, no 23, p. 6245-6262Article in journal (Refereed)
    Abstract [en]

    Experimental data from 593 photon beams were used to quantify the errors in dose calculations using a previously published pencil kernel model. A correction of the kernel was derived in order to remove the observed systematic errors. The remaining residual error for individual beams was modelled through uncertainty associated with the kernel model. The methods were tested against an independent set of measurements. No significant systematic error was observed in the calculations using the derived correction of the kernel and the remaining random errors were found to be adequately predicted by the proposed method.

  • 24.
    Nyholm, Tufve
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Olofsson, Jörgen
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Ahnesjö, Anders
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Modelling lateral beam quality variations in pencil kernel based photon dose calculations2006In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 51, no 16, p. 4111-4118Article in journal (Refereed)
    Abstract [en]

    Standard treatment machines for external radiotherapy are designed to yield flat dose distributions at a representative treatment depth. The common method to reach this goal is to use a flattening filter to decrease the fluence in the centre of the beam. A side effect of this filtering is that the average energy of the beam is generally lower at a distance from the central axis, a phenomenon commonly referred to as off-axis softening. The off-axis softening results in a relative change in beam quality that is almost independent of machine brand and model. Central axis dose calculations using pencil beam kernels show no drastic loss in accuracy when the off-axis beam quality variations are neglected. However, for dose calculated at off-axis positions the effect should be considered, otherwise errors of several per cent can be introduced. This work proposes a method to explicitly include the effect of off-axis softening in pencil kernel based photon dose calculations for arbitrary positions in a radiation field. Variations of pencil kernel values are modelled through a generic relation between half value layer (HVL) thickness and off-axis position for standard treatment machines. The pencil kernel integration for dose calculation is performed through sampling of energy fluence and beam quality in sectors of concentric circles around the calculation point. The method is fully based on generic data and therefore does not require any specific measurements for characterization of the off-axis softening effect, provided that the machine performance is in agreement with the assumed HVL variations. The model is verified versus profile measurements at different depths and through a model self-consistency check, using the dose calculation model to estimate HVL values at off-axis positions. A comparison between calculated and measured profiles at different depths showed a maximum relative error of 4% without explicit modelling of off-axis softening. The maximum relative error was reduced to 1% when the off-axis softening was accounted for in the calculations.

  • 25.
    Nyström, H
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Correction factors applied to plane-parallel ionization chambers1993In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 38, no 2, p. 311-322Article in journal (Refereed)
    Abstract [en]

    Plane-parallel ionization chambers are recommended by most dosimetry protocols' for use in low-energy electron beams. Such chambers are often calibrated at the clinic against a cylindrical chamber in a high-energy electron beam while no general procedures for Co-60 gamma-ray calibrations are yet adopted. If the perturbation due to the presence of the plane-parallel chamber in a water phantom at the calibration quality, i.e. Co-60, is known, a straightforward dose-to-water calibration would be possible. Furthermore, if the perturbation as a function of photon beam quality is known, the plane-parallel chamber might be used for all radiation qualities (both electrons and photons) used in megavoltage therapy. In this work the response of both plane-parallel and cylindrical chambers was studied and compared with ferrous sulphate dosimetry over a wide range of photon energies. Perturbation correction factors were determined for these chambers in photon beams. Also the influence of the aluminium central electrode of the NE2571 chamber was determined in the electron field relative to Co-60 gamma-radiation. Electron backscatter from the back wall of the chamber was shown to influence the signal in the chamber significantly. A procedure for calibration of Co-60 gamma-rays in water in plane-parallel chambers is discussed.

  • 26.
    Nyström, H
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Photon-beam quality specification by narrow-beam transmission measurements1994In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 39, no 8, p. 1231-1245Article in journal (Refereed)
    Abstract [en]

    Radiation quality specifications in megavoltage photon beams are usually based on depth-dose measurements performed under reference conditions. Stopping-power ratios and various correction factors are then related to parameters such as TPR10(20), which are extracted from depth-dose measurements. Stopping-power ratio determinations based on this concept were shown to be in error by more than 2% at high energies. Furthermore, electrons generated in the treatment bead can, at high energies, contribute to the dose at a depth of 10 cm and thus significantly affect the TPR10(20) ratio. This method was further shown to be inadequate when the dose in other parts of the field than the reference point was to be measured with ionization chamber dosimetry. A new standardized device for determining photon beam quality based on half value layer (HVL) measurements in water was developed and thoroughly investigated in both a low-energy, (4 MV) and a high-energy beam. A relation between HVL and stopping-power ratios water-to-air was determined by comparative measurements with air ionization chambers and liquid-filled ionization chambers together with Fricke dosimetry. Furthermore, different radiation quality gradients in the photon fields for different types of field-flattening systems, and field-compensating methods were discussed.

  • 27.
    Olofsson, Jörgen
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Nyholm, Tufve
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Ahnesjö, Anders
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Optimization of photon beam flatness for radiation therapy2007In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 52, no 6, p. 1735-1746Article in journal (Refereed)
    Abstract [en]

    In this work, we investigate the relation between lateral fluence/dose distributions and photon beam uniformity, possibly identifying ways to improve these characteristics. The calculations included treatment head scatter properties associated with three common types of linear accelerators in order to study their impact on the results. For 6 and 18 MV photon beams the lateral fluence distributions were optimized with respect to the resulting calculated flatness, as defined by the International Electrotechnical Commission (IEC), at 10 cm depth in six different field sizes. The limits proposed by IEC for maximum dose ratios ('horns') at the depth of dose maximum have also been accounted for in the optimization procedure. The conclusion was that typical head scatter variations among different types of linear accelerators have a very limited effect on the optimized results, which implies that the existing differences in measured off- axis dose distributions are related to non- equivalent optimization objectives. Finally, a comparison between the theoretically optimized lateral dose distributions and corresponding dose measurements for the three investigated accelerator types was performed. Although the measured data generally fall within the IECrequirements the optimized distributions show better results overall for the evaluated uniformity parameters, indicating that there is room for improved flatness performance in clinical photon beams.

  • 28.
    Olofsson, Lennart
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Magnus
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Effects on electron beam penumbra using the photon MLC to reduce bremsstrahlung leakage for an add-on electron MLC2005In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 50, no 6, p. 1191-1203Article in journal (Refereed)
    Abstract [en]

    Electron IMRT treatments have the potential to reduce the integral dose due to the limited range of the electrons. However, bremsstrahlung produced in the scattering foils could penetrate an added electron MLC (eMLC), thus producing an unmodulated dose contribution that could become unacceptable in electron IMRT treatments. To limit this bremsstrahlung contribution, the photon MLC (xMLC) was used to track the eMLC, but with a margin to avoid penumbra widening through partial screening of the effective electron source. The purpose of this work was to study the effect of the photon-electron MLC tracking on the electron beam penumbra for different treatment head designs. Both isocentric designs and designs where the eMLC is used close to the patient (proximity geometry) have been analysed using Monte Carlo simulations. At 22.5 MeV energy, a tracking margin of 1 cm was enough to avoid penumbra degradation for a helium-filled isocentric geometry, while air-filled geometries (including proximity geometries) require a 2-3 cm margin. Illustrated by an example of a chest wall treatment by electron IMRT, the use of 1 cm tracking margin will reduce the collimator leakage contribution by a factor of 36 as compared to using a static setting of the photon collimator.

  • 29.
    Puhakka, Pia
    et al.
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Ylärinne, Janne
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Lammi, Mikko
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Saarakkala, Simo
    Department of Medical Technology, Institute of Biomedicine, University of Oulu, Oulu, Finland.
    Tiitu, Virpi
    School of Medicine, Institute of Biomedicine, Anatomy, University of Eastern Finland, Kuopio, Finland.
    Kröger, Heikki
    Department of Orthopaedics, Traumatology and Hand Surgery, Kuopio University Hospital, Kuopio, Finland.
    Tuomas, Virén
    Cancer Center, Kuopio University Hospital, Kuopio, Finland.
    Jurvelin, Jukka
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Töyräs, Juha
    Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
    Dependence of light attenuation and backscattering on collagen concentration and chondrocyte density in agarose scaffolds2014In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 59, no 21, p. 6537-6548Article in journal (Refereed)
    Abstract [en]

    Optical coherence tomography (OCT) has been applied for high resolution imaging of articular cartilage. However, the contribution of individual structural elements of cartilage on OCT signal has not been thoroughly studied. We hypothesize that both collagen and chondrocytes, essential structural components of cartilage, act as important light scatterers and that variation in their concentrations can be detected by OCT through changes in backscattering and attenuation. To evaluate this hypothesis, we established a controlled model system using agarose scaffolds embedded with variable collagen concentrations and chondrocyte densities. Using OCT, we measured the backscattering coefficient (µb) and total attenuation coefficient (µt) in these scaffolds. Along our hypothesis, light backscattering and attenuation in agarose were dependent on collagen concentration and chondrocyte density. Significant correlations were found between µt and chondrocyte density (ρ = 0.853, p < 0.001) and between µt and collagen concentration (ρ = 0.694, p < 0.001). µb correlated significantly with chondrocyte density (ρ = 0.504, p < 0.001) but not with collagen concentration (ρ = 0.103, p = 0.422) of the scaffold. Thus, quantitation of light backscattering and, especially, attenuation could be valuable when evaluating the integrity of soft tissues, such as articular cartilage with OCT.

  • 30.
    Sätherberg, Anders
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Theoretical and experimental determination of phantom scatter factors for photon fields with different radial energy variation1996In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 41, no 12, p. 2687-2694Article in journal (Refereed)
    Abstract [en]

    The output factor used for monitor unit determination in radiotherapy can be divided into two factors: the head scatter factor and the phantom scatter factor. Theoretical and experimental phantom scatter factors have been compared for different beam qualities between 4 MV and 50 MV and field sizes from 5 cm x 5 cm to 30 cm x 30 cm. The theoretical data were obtained through a convolution method based on Monte Carlo calculated energy spectra and dose kernels. The calculations have been performed both for accelerators with a rather large energy variation within the field and for accelerators with a constant energy distribution in the field. Deviations between theoretical and experimental data were found to be less than 1%.

  • 31.
    Toma-Dasu, Iuliana
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Dasu, Alexandru
    Umeå University, Faculty of Medicine, Department of Radiation Sciences.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Conversion of polarographic electrode measurements – a computer based approach2005In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 50, no 19, p. 4581-4591Article in journal (Refereed)
    Abstract [en]

    The polarographic measurement of tissue oxygenation is one of the most widely used methods in clinical practice for the quantification of tumour hypoxia. However, due to the particular features of the electrode measuring process, the results of the measurements do not accurately reflect the tumour oxygenation. This study aimed to find a correlation between the electrode measurements and the tumour oxygenation in an attempt to improve the accuracy of the predictions regarding the response to treatment based on electrode measurements. A previously developed computer model that allows the simulation of tumour tissue and electrode measurements was used. The oxygenation of a large number of tumours with biologically relevant distributions of blood vessels was theoretically calculated. Simulations of electrode measurements allowed the comparison between the real tissue oxygenation and the results obtained with the electrode. A semi-empirical relationship between the hypoxic fraction measured by the electrode and the real hypoxic fraction in the tissue has been found. The impact of the correction of the electrode measurements in terms of predictions for tumour control probability was estimated for a few clinical examples. The range of possible true values corresponding to one measurement has also proven useful for explaining the apparently unexpected response to the treatment of some patients. The corrected hypoxic fraction which is believed to be closer to the real value of tissue hypoxia predicts much smaller control probabilities than the raw electrode measurements. This could provide an explanation for the apparently unexpected failure to respond to the treatment of some of the patients with apparently favourable tumour oxygenation. This also means that the electrode measurements cannot be used directly for the quantitative modelling of tumour response to the treatment. The conversion method proposed in this paper might however strengthen the statistical power of the correlations between the electrode measurements and the treatment outcome.

  • 32.
    Toma-Dasu, Iuliana
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Dasu, Alexandru
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    The relationship between temporal variation of hypoxia, polarographic measurements and predictions of tumour response to radiation2004In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 49, no 19, p. 4463-4475Article in journal (Refereed)
    Abstract [en]

    The polarographic oxygen sensor is one of the most used devices for in vivo measurements of oxygen and many other measurement techniques for measuring tumour hypoxia are correlated with electrode measurements. Little is known however about the relationship between electrode measurements and the real tissue oxygenation. This paper investigates the influence of the temporal change of the hypoxic pattern on the electrode measurements and the tumour response.

    Electrode measurements and tumour response were simulated using a computer program that allows both the calculation of the tissue oxygenation with respect to the two types of hypoxia that might arise in tumours and the virtual insertion of the electrode into the tissue. It was therefore possible to control the amount of each type of hypoxia in order to investigate their influence on the measurement results. Tissues with several vascular architectures ranging from well oxygenated to poorly oxygenated were taken into consideration as might be seen in practice. The influence of the electrode measurements on the treatment outcome was estimated by calculating the tumour control probability for the tumours characterized either by the real or by the measured tumour oxygenation.

    We have simulated electrode oxygen measurements in different types of tissues, covering a wide range of tumour oxygenations. The results of the simulations showed that the measured distribution depends on the details of the vascular network and not on the type of hypoxia. We have also simulated the effects of the temporal change of the acute hypoxic pattern due to the opening and the closure of different blood vessels during a full fractionated treatment. The results of this simulation suggested that the temporal variation of the hypoxic pattern does not lead to significantly different results for the electrode measurements or the predicted tumour control probabilities.

    In conclusion, it was found that the averaging effect of the electrode leads to a systematic deviation between the actual oxygen distribution and the measured distribution. However, as the electrode reflects the general trends of the tissue oxygenation it has the potential of being used for the general characterization of tumour hypoxia even if the actual type of hypoxia measured by the electrode cannot be determined. Indeed, the change in time of the acute hypoxic region does not compensate for the lack of oxygenation at a specific moment and therefore does not influence the polarographic oxygen measurements.

  • 33.
    Tölli, Heikki
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Sjögren, Rickard
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Wendelsten, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    A two-dose-rate method for general recombination correction for liquid ionization chambers in pulsed beams.2010In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 15, p. 4247-4260Article in journal (Refereed)
    Abstract [en]

    The correction for general recombination losses in liquid ionization chambers (LICs) is more complex than that in air-filled ionization chambers. The reason for this is that the saturation charge in LICs, i.e. the charge that escapes initial recombination, depends on the applied voltage. This paper presents a method, based on measurements at two different dose rates in a pulsed beam, for general recombination correction in LICs. The Boag theory for pulsed beams is used and the collection efficiency is determined by numerical methods which are equivalent to the two-voltage method used in dosimetry with air-filled ionization chambers. The method has been tested in experiments in water in a 20 MeV electron beam using two LICs filled with isooctane and tetramethylsilane. The dose per pulse in the electron beam was varied between 0.1 mGy/pulse and 8 mGy/pulse. The relative standard deviations of the collection efficiencies determined with the two-dose-rate method ranged between 0.1% and 1.5%. The dose-rate variations of the general recombination corrected charge measured with the LICs are in excellent agreement with the corresponding values obtained with an air-filled plane parallel ionization chamber.

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