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Fast modelling of spectra and stopping-power ratios using differentiated fluence pencil kernels
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Oncology.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Oncology.
2008 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 53, no 16, 4231-4247 p.Article in journal (Refereed) Published
Abstract [en]

Modern radiotherapy steadily utilizes more of the available degrees of freedom provided by radiotherapy equipment, raising the need for the dosimetric methods to deliver reliable measurements for situations where the spectral properties of the radiation field may also vary. A kernel-based superposition method is presented for which the spectra from any field modulation can be instantly calculated, thus facilitating the determination of dosimetric quantities at arbitrary locations. A database of fluence pencil kernels describing the fluence resulting from point monodirectional monoenergetic beams incident onto a water phantom has been calculated with the PENELOPE-2005 Monte Carlo package. Spectra calculated by means of the kernels are presented for various 6 MV fields. The spectra have been used to investigate depth and lateral variations of water-to-air stopping-power ratios. Results show that the stopping-power ratio decreases with depth, and that this effect is more pronounced for small fields. These variations are clearly connected to spectral variations. For a 10 x 10 cm(2) field, the difference between the stopping-power ratio at 2.5 cm depth and 30 cm depth is less than 0.3% while for a 0.3 x 0.3 cm(2) field this difference is 0.7%. Ratios outside the field were found to be sensitive to the collimator leakage spectral variations.

Place, publisher, year, edition, pages
2008. Vol. 53, no 16, 4231-4247 p.
National Category
Medical and Health Sciences
URN: urn:nbn:se:uu:diva-103245DOI: 10.1088/0031-9155/53/16/002ISI: 000258144300003PubMedID: 18653924OAI: oai:DiVA.org:uu-103245DiVA: diva2:217787
Available from: 2009-05-15 Created: 2009-05-15 Last updated: 2010-05-31Bibliographically approved
In thesis
1. Modeling Silicon Diode Dose Response in Radiotherapy Fields using Fluence Pencil Kernels
Open this publication in new window or tab >>Modeling Silicon Diode Dose Response in Radiotherapy Fields using Fluence Pencil Kernels
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In radiotherapy, cancer is treated with ionizing radiation, most commonly bremsstrahlung photons from electrons of several MeV. Secondary electrons produced in photon-interactions results in dose deposition. The treatment response is low for low doses, raises sharply for normal treatment doses and saturates at higher doses. This response pattern applies to both eradication of tumors and to complications in healthy tissues. Well controlled treatments require accurate dosimetry since the uncertainty in delivered dose will be magnified 1 to 5 times in treatment response variations. Techniques that superpose many small radiation fields to concentrate the dose to a localized target are becoming increasingly used. A detector with high spatial resolution suitable for such fields is a silicon diode. To maintain the current accuracy of the dosimetric calibration of 1.5%, diode measurements relative to this calibration should preferably be possible at 0.5% accuracy level.

The main limitation of silicon diodes is their over-response to low-energy photons. This problem has been adressed with the insertion of a high atomic number filter in diodes. For modeling diode detector response one must quantify the spectral variations in the irradiated medium resulting from variations of the beam parameters. This requires understanding of the particle transport and can be achieved by Monte Carlo simulations. However, the small dimensions of the detector geometry compared to surrounding medium makes a direct application of Monte Carlo impractical due to the large amount of CPU time necessary to reach statistically satisfactory results.

In this work a fast method for spectra calculations is used, based on superposition of mono-energetic fluence pencil kernels. Building on this base a general model for silicon response functions in photon fields is developed. The incident photons are bipartitioned into a low and a high energy component. The high energy part is treated with the Spencer-Attic cavity theory while the low energy part and scattered photons are treated with large cavity theory. The deviations from electron equilibrium are investigated and handled with correction factors. The result is used to correct unshielded diode measurements, with an overall uncertainty less than 0.5%, except for very small fields where the precision is around 1-2%, thus eliminating the need for less predictable shielded diodes for measurements in photon fields.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 48 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 536
National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
Medical Radiophysics
urn:nbn:se:uu:diva-120581 (URN)978-91-554-7748-6 (ISBN)
Public defence
2010-04-29, Skoogsalen, ingång 78, Akademiska sjukhuset, Uppsala, 13:00 (English)
Available from: 2010-04-08 Created: 2010-03-15 Last updated: 2010-04-08Bibliographically approved

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