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Verification of dose calculations in radiotherapy
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

External radiotherapy is a common treatment technique for cancer. It has been shown that radiation therapy is a both clinically and economically effective treatment for many types of cancer, even though the equipment is expensive. The technology is in constant evolution and more and more sophisticated and complex techniques are introduced. One of the main tasks for physicists at a radiotherapy department is quality control, i.e. making sure that the treatments are delivered in accordance with the dosimetric intentions. Over dosage of radiation can lead to severe side effects, while under dosage reduces the probability for patient cure.

The present thesis is mainly focused on the verification of the calculated dose. Requirements for independent dose calculation software are identified and the procedures using such software are described. In the publications included in the thesis an algorithm specially developed for verification of dose calculations is described and tested. The calculation uncertainties connected with the described algorithm are investigated and modeled. A brief analysis of the quality assurance procedures available and used in external radiotherapy is also included in the thesis.

The main conclusion of the thesis is that independent verification of the dose calculations is feasible in an efficient and cost effective quality control system. The independent calculations do not only serve as a protection against accidents, but can also be the basis for comparisons of the dose calculation performance at different clinics.

Place, publisher, year, edition, pages
Umeå: Strålningsvetenskaper , 2008. , 63 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1224
Keyword [en]
radiotherapy, dose calculation, quality control, pencil kernel
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:umu:diva-1931ISBN: 978‐91‐7264‐679‐7 OAI: oai:DiVA.org:umu-1931DiVA: diva2:142477
Public defence
2008-12-12, 244, By 7, Norrlands universitetssjukhus, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2008-11-24 Created: 2008-11-24 Last updated: 2012-04-02Bibliographically approved
List of papers
1. Photon pencil kernel parameterisation based on beam quality index
Open this publication in new window or tab >>Photon pencil kernel parameterisation based on beam quality index
2006 (English)In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 78, no 3, 347-351 p.Article in journal (Refereed) Published
Abstract [en]

Background and purpose: New treatment techniques in radiotherapy employ increasing dose calculation complexity in treatment planning. For an adequate check of the results coming from a modern treatment planning system, clinical tools with almost the same degree of generality and accuracy as the planning system itself are needed. To fulfil this need we propose a photon pencil kernel parameterization based on a minimum of input data that can be used for phantom scatter calculations. Through scatter integration the pencil kernel model can calculate common parameters, such as TPR or phantom scatter factors, used in various dosimetric QA (quality assurance) procedures. Material and methods: The proposed model originates from an already published radially parameterized pencil kernel. A depth parameterization of the pencil kernel parameters has been introduced, based on a large database containing commissioned beam data for a commercial treatment planning system. The entire pencil kernel model demands only one photon beam quality index, TPR20,10, as input. Results: By comparing the dose calculation results to the extensive experimental data set in the database, it has been possible to make a thorough analysis of the resulting accuracy. The errors in calculated doses, normalized to the reference geometry, are in most cases smaller than 2%. Conclusions: The investigation shows that a pencil kernel model based only on TPR20,10 can be used for dosimetric verification purposes in megavoltage photon beams at depths below the range of contaminating electrons. (c) 2006 Elsevier Ireland Ltd.

Identifiers
urn:nbn:se:umu:diva-13486 (URN)10.1016/j.radonc.2006.02.002 (DOI)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2017-12-14Bibliographically approved
2. Modelling lateral beam quality variations in pencil kernel based photon dose calculations
Open this publication in new window or tab >>Modelling lateral beam quality variations in pencil kernel based photon dose calculations
2006 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 51, no 16, 4111-4118 p.Article in journal (Refereed) Published
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.

Identifiers
urn:nbn:se:umu:diva-13398 (URN)10.1088/0031-9155/51/16/016 (DOI)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2017-12-14Bibliographically approved
3. Pencil kernel correction and residual error estimation for quality-index-based dose calculations
Open this publication in new window or tab >>Pencil kernel correction and residual error estimation for quality-index-based dose calculations
Show others...
2006 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 51, no 23, 6245-6262 p.Article in journal (Refereed) Published
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.

Identifiers
urn:nbn:se:umu:diva-15748 (URN)10.1088/0031-9155/51/23/021 (DOI)17110783 (PubMedID)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2017-12-14Bibliographically approved
4. Evaluation of uncertainty predictions and dose output for model-based dose calculations for megavoltage photon beams
Open this publication in new window or tab >>Evaluation of uncertainty predictions and dose output for model-based dose calculations for megavoltage photon beams
Show others...
2006 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 7, 2548-2556 p.Article in journal (Refereed) Published
Abstract [en]

In many radiotherapy clinics an independent verification of the number of monitor units (MU) used to deliver the prescribed dose to the target volume is performed prior to the treatment start. Traditionally this has been done by using methods mainly based on empirical factors which, at least to some extent, try to separate the influence from input parameters such as field size, depth, distance, etc. The growing complexity of modern treatment techniques does however make this approach increasingly difficult, both in terms of practical application and in terms of the reliability of the results. In the present work the performance of a model-based approach, describing the influence from different input parameters through actual modeling of the physical effects, has been investigated in detail. The investigated model is based on two components related to megavoltage photon beams; one describing the exiting energy fluence per delivered MU, and a second component describing the dose deposition through a pencil kernel algorithm solely based on a measured beam quality index. Together with the output calculations, the basis of a method aiming to predict the inherent calculation uncertainties in individual treatment setups has been developed. This has all emerged from the intention of creating a clinical dose/MU verification tool that requires an absolute minimum of commissioned input data. This evaluation was focused on irregular field shapes and performed through comparison with output factors measured at 5, 10, and 20 cm depth in ten multileaf collimated fields on four different linear accelerators with varying multileaf collimator designs. The measurements were performed both in air and in water and the results of the two components of the model were evaluated separately and combined. When compared with the corresponding measurements the resulting deviations in the calculated output factors were in most cases smaller than 1% and in all cases smaller than 1.7%. The distribution describing the calculation errors in the total dose output has a mean value of -0.04% and a standard deviation of 0.47%. In the dose calculations a previously developed correction of the pencil kernel was applied that managed to contract the error distribution considerably. A detailed analysis of the predicted uncertainties versus the observed deviations suggests that the predictions indeed can be used as a basis for creating action levels and tracking dose calculation errors in homogeneous media. (C) 2006 American Association of Physicists in Medicine.

Identifiers
urn:nbn:se:umu:diva-13388 (URN)10.1118/1.2207316 (DOI)16898459 (PubMedID)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2017-12-14Bibliographically approved

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