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Dose uncertainties in photon pencil kernel calculations at off-axis positions
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: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 9, 3418-3425 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study was to investigate the specific problems associated with photon dose calculations in points located at a distance from the central beam axis. These problems are related to laterally inhomogeneous energy fluence distributions and spectral variations causing a lateral shift in the beam quality, commonly referred to as off-axis softening (OAS). We have examined how the dose calculation accuracy is affected when enabling and disabling explicit modeling of these two effects. The calculations were performed using a pencil kernel dose calculation algorithm that facilitates modeling of OAS through laterally varying kernel properties. Together with a multisource model that provides the lateral energy fluence distribution this generates the total dose output, i.e., the dose per monitor unit, at an arbitrary point of interest. The dose calculation accuracy was evaluated through comparisons with 264 measured output factors acquired at 5, 10, and 20 cm depth in four different megavoltage photon beams. The measurements were performed up to 18 cm from the central beam axis, inside square fields of varying size and position. The results show that calculations including explicit modeling of OAS were considerably more accurate, up to 4%, than those ignoring the lateral beam quality shift. The deviations caused by simplified head scatter modeling were smaller, but near the field edges additional errors close to 1% occurred. When enabling full physics modeling in the dose calculations the deviations display a mean value of -0.1%, a standard deviation of 0.7%, and a maximum deviation of -2.2%. Finally, the results were analyzed in order to quantify and model the inherent uncertainties that are present when leaving the central beam axis. The off-axis uncertainty component showed to increase with both off-axis distance and depth, reaching 1% (1 standard deviation) at 20 cm depth. (c) 2006 American Association of Physicists in Medicine.

Place, publisher, year, edition, pages
2006. Vol. 33, no 9, 3418-3425 p.
Keyword [en]
dosimetry, physiological models, radiation therapy, dose calculation, uncertainties, off-axis, pencil kernel, head scatter
URN: urn:nbn:se:umu:diva-13322DOI: 10.1118/1.2335488PubMedID: 17022238OAI: diva2:152993
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2011-03-25Bibliographically 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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