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Modeling Silicon Diode Dose Response in Radiotherapy Fields using Fluence Pencil Kernels
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Avdelningen för sjukhusfysik.
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. , p. 48
Series
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
Identifiers
URN: urn:nbn:se:uu:diva-120581ISBN: 978-91-554-7748-6 (print)OAI: oai:DiVA.org:uu-120581DiVA, id: diva2:303663
Public defence
2010-04-29, Skoogsalen, ingång 78, Akademiska sjukhuset, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2010-04-08 Created: 2010-03-15 Last updated: 2010-04-08Bibliographically approved
List of papers
1. Fast modelling of spectra and stopping-power ratios using differentiated fluence pencil kernels
Open this publication in new window or tab >>Fast modelling of spectra and stopping-power ratios using differentiated fluence pencil kernels
2008 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 53, no 16, p. 4231-4247Article 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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-103245 (URN)10.1088/0031-9155/53/16/002 (DOI)000258144300003 ()18653924 (PubMedID)
Available from: 2009-05-15 Created: 2009-05-15 Last updated: 2017-12-13Bibliographically approved
2. Modeling silicon diode energy response factors for use in therapeutic photon beams
Open this publication in new window or tab >>Modeling silicon diode energy response factors for use in therapeutic photon beams
2009 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 54, no 20, p. 6135-6150Article in journal (Refereed) Published
Abstract [en]

Silicon diodes have good spatial resolution, which makes them advantageous over ionization chambers for dosimetry in fields with high dose gradients. However, silicon diodes overrespond to low-energy photons, that are more abundant in scatter which increase with large fields and larger depths. We present a cavity-theory-based model for a general response function for silicon detectors at arbitrary positions within photon fields. The model uses photon and electron spectra calculated from fluence pencil kernels. The incident photons are treated according to their energy through a bipartition of the primary beam photon spectrum into low- and high-energy components. Primary electrons from the high-energy component are treated according to Spencer–Attix cavity theory. Low-energy primary photons together with all scattered photons are treated according to large cavity theory supplemented with an energy-dependent factor K(E) to compensate for energy variations in the electron equilibrium. The depth variation of the response for an unshielded silicon detector has been calculated for 5 × 5 cm2, 10 × 10 cm2 and 20 × 20 cm2 fields in 6 and 15 MV beams and compared with measurements showing that our model calculates response factors with deviations less than 0.6%. An alternative method is also proposed, where we show that one can use a correlation with the scatter factor to determine the detector response of silicon diodes with an error of less than 3% in 6 MV and 15 MV photon beams.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-120550 (URN)10.1088/0031-9155/54/20/007 (DOI)000270563300007 ()19779220 (PubMedID)
Available from: 2010-03-12 Created: 2010-03-12 Last updated: 2017-12-12Bibliographically approved
3. Modeling silicon diode dose response factors for small photon fields
Open this publication in new window or tab >>Modeling silicon diode dose response factors for small photon fields
2010 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 24, p. 7411-7423Article 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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-120594 (URN)10.1088/0031-9155/55/24/002 (DOI)000284825200002 ()21098913 (PubMedID)
Available from: 2010-03-15 Created: 2010-03-15 Last updated: 2017-12-12Bibliographically approved
4. Spectral perturbations from silicon diode detector encapsulation and shielding in photon fields
Open this publication in new window or tab >>Spectral perturbations from silicon diode detector encapsulation and shielding in photon fields
2010 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 37, no 11, p. 6055-6060Article in journal (Refereed) Published
Abstract [en]

PURPOSE

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.

METHOD

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.

RESULTS

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.

CONCLUSIONS

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.

Keywords
silicone diode; photon spectra; detector response
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-120602 (URN)10.1118/1.3501316 (DOI)000283747600051 ()
Available from: 2010-03-15 Created: 2010-03-15 Last updated: 2017-12-12Bibliographically approved
5. Detector response modeling
Open this publication in new window or tab >>Detector response modeling
Show others...
2009 (English)Patent (Other (popular science, discussion, etc.))
Abstract [en]

A detector response correction arrangement and method is proposed for online determination of correction factors for arbitrary positions from arbitrary incident fluence distributions. As modern radiotherapy utilizes more of the available degrees of freedom of radiation machines, dosimetry has to be able to present reliable measurements for all these degrees of freedom. To determine correction factors online during measurement, Monte Carlo technique is used to precalculate fluence pencil kernels from a monodirectional beam to fully describe the particle fluence in an irradiated medium. Assuming that the particle fluence is not significantly altered by the introduction of a small detector volume, the fluence pencil kernels (212) can be integrated (214), and correction factors (216) determined, e.g. by Cavity Theory, in different positions for the detector material.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-120609 (URN)
Patent
US US2009090870 (A1) (2009-04-09)
Available from: 2010-03-15 Created: 2010-03-15 Last updated: 2012-05-29Bibliographically approved

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