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A widely tested model for head scatter influence on photon beam output
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
2003 (English)In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 67, no 2, 225-238 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: To construct and test a semi-analytical model describing the effects on Monitor Unit (MU) verification caused by scattering in the treatment head. The implementation of the model should be accomplished using a small set of experimental data. Furthermore, the model should include a geometry dependent estimation of the resulting uncertainty.

Material and methods: The input required by the created model consists of basic treatment head geometry and 10 measured output factors in air (OFair) for square fields. It considers primary energy fluence, scattered radiation from an extra-focal source and from secondary collimators, as well as backscatter to the monitor chamber. Measurements and calculations were performed in open symmetric and asymmetric fields at points located both on and off the collimator axis, as well as at arbitrary treatment distances. The model has been verified for 19 photon beams in the range from 4 up to 50 MV, provided by nine different treatment units from six manufacturers.

Results: The presented model provided results with errors smaller than 1% (2 S.D.) in typical clinical situations for all beams tested. In more exceptional situations, i.e. combinations of unconventional treatment head designs, very elongated fields, and dosimetry points far away from the isocenter, the total uncertainty increased to approximately 2%. The spread in the results was further analysed in order to create a method for predicting the uncertainties under different treatment conditions.

Conclusions: A general head scatter model that is easy to implement has been developed and can be used as the basis for computerised MU verification. The model handles all commercially available treatment units adequately and also includes an estimation of the resulting uncertainty.

Place, publisher, year, edition, pages
2003. Vol. 67, no 2, 225-238 p.
National Category
Cancer and Oncology
URN: urn:nbn:se:umu:diva-5253DOI: 10.1016/S0167-8140(02)00409-7PubMedID: 12812855OAI: diva2:144707
Available from: 2006-08-31 Created: 2006-08-31 Last updated: 2012-06-26Bibliographically approved
In thesis
1. Developing and evaluating dose calculation models for verification of advanced radiotherapy
Open this publication in new window or tab >>Developing and evaluating dose calculation models for verification of advanced radiotherapy
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A prerequisite for modern radiotherapy is the ability to accurately determine the absorbed dose (D) that is given to the patient. The subject of this thesis has been to develop and evaluate efficient dose calculation models for high-energy photon beams delivered by linear accelerators. Even though the considered calculation models are general, the work has been focused on quality assurance (QA) tools used to independently verify the dose for individual treatment plans. The purpose of this verification is to guarantee patient safety and to improve the treatment outcome. Furthermore, a vital part of this work has been to explore the prospect of estimating the dose calculation uncertainties associated with individual treatment setups. A discussion on how such uncertainty estimations can facilitate improved clinical QA procedures by providing appropriate action levels has also been included within the scope of this thesis.

In order to enable efficient modelling of the physical phenomena that are involved in dose output calculations it is convenient to divide them into two main categories; the first one dealing with the radiation exiting the accelerator’s treatment head and a second one associated with the subsequent energy deposition processes. A multi-source model describing the distribution of energy fluence emitted from the treatment head per delivered monitor unit (MU) is presented and evaluated through comparisons with measurements in multiple photon beams and collimator settings. The calculations show close agreement with the extensive set of experimental data, generally within +/-1% of corresponding measurements.

The energy (dose) deposition in the irradiated object has been modelled through a photon pencil kernel solely based on a beam quality index (TPR20,10). This model was evaluated in a similar manner as the multi-source model at three different treatment depths. A separate study was focused on the specific difficulties associated with dose calculations in points located at a distance from the central beam axis. Despite the minimal input data required to characterize individual photon beams, the accuracy proved to be very good when comparing the calculated results with experimental data.

The evaluated calculation models were finally used to analyse how well the lateral dose distributions from typical megavoltage photon beams are optimized with respect to the resulting beam flatness characteristics. The results did not reveal any obvious reasons why different manufacturers should provide different lateral dose distributions. Furthermore, the performed lateral optimizations indicate that there is room for improved flatness performance for the investigated linear accelerators.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2006. 50 p.
Umeå University medical dissertations, ISSN 0346-6612 ; 1044
Radiation therapy, high-energy photons, dose calculation, multi-source model, pencil kernel, uncertainties, action levels
National Category
Radiology, Nuclear Medicine and Medical Imaging
urn:nbn:se:umu:diva-841 (URN)91-7264-141-X (ISBN)
Public defence
2006-09-22, 244, 7, Norrlands universitetssjukhus, Umeå, 13:00 (English)
Available from: 2006-08-31 Created: 2006-08-31 Last updated: 2012-04-03Bibliographically approved

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