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Modelling lateral beam quality variations in pencil kernel based photon dose calculations
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
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.

Place, publisher, year, edition, pages
2006. Vol. 51, no 16, 4111-4118 p.
URN: urn:nbn:se:umu:diva-13398DOI: 10.1088/0031-9155/51/16/016OAI: diva2:153069
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2011-03-25Bibliographically approved
In thesis
1. Verification of dose calculations in radiotherapy
Open this publication in new window or tab >>Verification of dose calculations in radiotherapy
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.
Umeå University medical dissertations, ISSN 0346-6612 ; 1224
radiotherapy, dose calculation, quality control, pencil kernel
National Category
Radiology, Nuclear Medicine and Medical Imaging
urn:nbn:se:umu:diva-1931 (URN)978‐91‐7264‐679‐7 (ISBN)
Public defence
2008-12-12, 244, By 7, Norrlands universitetssjukhus, Umeå, 09:00 (English)
Available from: 2008-11-24 Created: 2008-11-24 Last updated: 2012-04-02Bibliographically approved

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Nyholm, TufveOlofsson, JörgenAhnesjö, AndersKarlsson, Mikael
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