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Spectral perturbations from silicon diode detector encapsulation and shielding in photon fields
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.
2010 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 37, no 11, 6055-6060 p.Article in journal (Refereed) Published
Abstract [en]


Silicon diodes are widely used as detectors for relative dose measurements in radiotherapy. Generally two types of diode mountings are used. Plastic encapsulation is used for electron fields while the encapsulation for diodes intended for photon fields include a shield of high density material (typically tungsten). The purpose of the shield is to absorb low energy scattered photons to which a silicon diode over-responses. However, new models based on spectra calculations have been proposed for direct correction of the readout from unshielded (e.g.”electron”) diodes in photon fields. This raises the question whether it is correct to assume that the spectrum calculated for water is not disturbed by the detector encapsulation. This work aims at investigating the spectral effects of the encapsulation materials typical for typical silicon diodes used in radiotherapy clinics, including the effects of the shielding traditionally used for photon field diodes.


The effects of detector encapsulation of an unshielded and a shielded commercial diode on the spectra at the silicon chip location are studied through Monte Carlo simulations with PENELOPE-2005. Variance reduction based on importance sampling and correlated sampling is applied to reduce the CPU-time needed for the simulations.


The use of variance reduction is proved to be efficient and to not introduce any significant bias of the results. Compared to reference spectra calculated in water, the encapsulation for an unshielded diode is demonstrated to not perturb the spectrum while tungsten shielded diode caused not only the desired decrease in low energy scattered photons but also a large increase of primary electrons of all energies. Measurements with a shielded diode in a 6MV photon beam proved that the shielding does not completely remove the field-size dependence of the detector response caused by the over response from low energy photons.


Spectra calculated for water can be directly used for modeling the response of silicon diodes with plastic only encapsulations. For photon dose measurements, an unshielded diode used together with appropriate corrections gives more accurate results than the traditionally used shielded diodes. Variance reduction for diode simulations can effectively be applied, however with great considerations considering choice of application.

Place, publisher, year, edition, pages
2010. Vol. 37, no 11, 6055-6060 p.
Keyword [en]
silicone diode; photon spectra; detector response
National Category
Medical and Health Sciences
URN: urn:nbn:se:uu:diva-120602DOI: 10.1118/1.3501316ISI: 000283747600051OAI: oai:DiVA.org:uu-120602DiVA: diva2:303653
Available from: 2010-03-15 Created: 2010-03-15 Last updated: 2010-12-08Bibliographically 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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