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Modeling silicon diode dose response factors for small photon fields
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Section of Medical Physics. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
2010 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 24, 7411-7423 p.Article in journal (Refereed) Published
Abstract [en]

The dosimetry of small fields is important for the use of high resolution photon radiotherapy. Silicon diodes yield a high signal from a small detecting volume which makes them suitable for use in small fields and high dose gradients. Unshielded diodes used in large fields are known to give a varying dose response depending on the proportion of low energy scattered photons in the field. Response variations in small fields can be caused by both spectral variations, and disturbances of the local level of lateral electron equilibrium. We present a model that includes the effects from lack of charged particle equilibrium. The local spectra are calculated by use of fluence pencil kernels and divided into a low and a high energy component. The low energy part is treated with large cavity theory and the high energy part with the Spencer-Attix small cavity theory. Monte Carlo-derived correction factors are used to account for both the local level of electron equilibrium in the field, and deviations from this level in the silicon disk cavity. Results for field sizes ranging from 0.5 × 0.5 to 20 × 20 cm2 are compared to data from full Monte Carlo simulations and measurements. The achieved dose response accuracy is for the smallest fields 1-2%, and for larger fields 0.5%. Spectral variations were of little importance for the small field response, implying that volume averaging, and to some extent interface transient effects, are of importance for use of unshielded diodes in non-equilibrium conditions. The results indicate that diodes should preferably be designed to have the thin layer of active volume padded in between inactive layers of the silicon base material.

Place, publisher, year, edition, pages
2010. Vol. 55, no 24, 7411-7423 p.
National Category
Medical and Health Sciences
URN: urn:nbn:se:uu:diva-120594DOI: 10.1088/0031-9155/55/24/002ISI: 000284825200002PubMedID: 21098913OAI: oai:DiVA.org:uu-120594DiVA: diva2:303645
Available from: 2010-03-15 Created: 2010-03-15 Last updated: 2011-11-10Bibliographically 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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