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  • 1.
    Karlsson, Magnus G
    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.
    Sjögren, Rickard
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Svensson, Hans
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Semi-conductor detectors in output factor measurements1997In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 42, no 3, p. 293-296Article in journal (Refereed)
    Abstract [en]

    Background and purpose: Output factors are generally measured with cylindrical ionization chambers. It was investigated if Si-diodes of p-type instead could be used. The advantage would be the small detector size and the robust construction of the detector.

    Materials and methods: Two types of diodes were studied, one with a shielding layer of tungsten specially made to reduce the excess response for scattered photons and one standard diode without any extra shielding. The measurements were performed at accelerating potentials between 4 and 50 MV and beam sizes between 4 cm x 4 cm and 40 cm x 40 cm.

    Results: The results showed that both types of diodes are suitable for measurements of head scatter factors in mini-phantoms. However, the diodes were found inappropriate for measurement of output factors for large fields in extended water phantoms. For small fields (<10 cm x 10 cm) a small detector is advantageous and no errors due to the scatter contribution were seen.

    Conclusions: An cylindrical ionization chamber is the best choice for output factor measurements in extended water phantoms for large field sizes while diodes are an alternative in small fields. There were negligible differences between the detectors in head scatter measurements in mini phantoms.

  • 2.
    Mu, Xiangkui
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
    Olofsson, Lennart
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Mikael
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Sjögren, Rickard
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Zackrisson, Björn
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
    Can photon IMRT be improved by combination with mixed electron and photon techniques?2004In: Acta Oncologica, ISSN 0284-186X, E-ISSN 1651-226X, Vol. 43, no 8, p. 727-735Article in journal (Refereed)
    Abstract [en]

    Conformal radiotherapy or intensity modulated radiotherapy (IMRT) commonly leads to a large integral dose in the patient. Electrons would reduce the integral dose but are not suitable for treating deep-seated tumours, owing to their limited penetration. By combining electron and photon beams, the dose distributions may be improved. In this study, the possibility is explored of using a mixture of electron and photon beams for a deep-seated target volume in the head and neck region. Treatment plans were made for five simulated head and neck cancer cases. Mixed electron and photon beam plans (MB) were constructed using a manual iterative procedure. Photon IMRT plans were optimized automatically. Both electron and photon beams were collimated by a computer controlled multi-leaf collimator (MLC). Both methods were able to produce clinically acceptable plans. Criteria for the target dose were met similarly by both as were the criteria for critical organs. The integral dose outside the planning target volume (PTV) showed a tendency to be lower with MB plans compared with photon IMRT plans. A mixed electron and photon technique has the potential to treat deep-seated tumours. It is reasonable to expect that if computerized optimization tools were coupled with the mixed electron and photon beam technique, treatment goals would be more readily achieved than if using solely pure photon IMRT.

  • 3.
    Sjögren, Rickard
    et al.
    Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
    Karlsson, Magnus G
    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.
    Depth for dose calibration in high energy photon beams1997In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 43, no 3, p. 311-313Article in journal (Refereed)
    Abstract [en]

    Background and purpose: The normalisation depth for determination of output factors in photon fields has frequently been the depth of dose maximum. At high energies the contribution from contaminating electrons is significant at dose maximum and is critically dependent on the beam geometry parameters, which is why a larger depth should be preferred.

    Materials and methods: The effect of electron contamination was studied using a purging magnet to remove charged particles from the treatment head and a helium bag to minimise production between the head and the phantom.

    Results: A depth of 10 cm was found to be beyond the range of the contaminating electrons for photon energies up to 20 MV (TPR1020 = 0.772). However, at 50 MV (TPR1020 = 0.810) contaminating electrons contribute 2-3% to the absorbed dose at 10 cm depth.

    Conclusions: 10 cm is recommended as both reference and normalisation depth for all megavoltage photon beam qualities, i.e. Co-60 and X-rays from accelerators up to 50 MV. (C) 1997 Elsevier Science Ireland Ltd.

  • 4.
    Sjögren, Rickard
    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.
    Electron contamination in clinical high energy photon beams1996In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 23, no 11, p. 1873-1881Article in journal (Refereed)
    Abstract [en]

    The electron contamination in photon beams has been investigated by means of contaminating lepton depth doses and dose profiles in different geometries with two 20 MV beams. Different components of this contamination have been investigated separately by systematically adding contamination to a ''clean'' reference field. At 20 MV, the air generated electrons were found to be almost negligible compared to the electrons originating from the accelerator head when measurements were performed in standard fields at SSDs between 80 and 120 cm. The total electron part of the depth dose curve was then almost the same, i.e., independent of SSD, when the collimator opening was held fixed. However, when different accessories such as a shaping block and different attenuating plates were located in the beam path below the collimators, a large SSD dependence of the electron contamination was noticed. A comparison was also made between two machines, one equipped with a multileaf collimator, with similar beam qualities at 20 MV. These measurements indicate that the interior view of the treatment head seen by the detector (mainly the flattening filter, monitor chamber, or other electron generating material) influences the magnitude of the electron contamination. When the collimator opening is decreased the electron contamination will also decrease as parts of the electron source will be shielded by the collimator blocks.

  • 5.
    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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