By a News Reporter-Staff News Editor at Health & Medicine Week -- Current study results on Health and Medicine have been published. According to news originating from Sydney, Australia, by NewsRx correspondents, research stated, "Current amorphous silicon electronic portal imaging devices (a-Si EPIDs) that are frequently used in radiotherapy applications employ a metal plate/phosphor screen configuration to optimize x-ray detection efficiency. The phosphor acts to convert x rays into an optical signal that is detected by an underlying photodiode array."
Our news journalists obtained a quote from the research from the University of Sydney, "The dosimetric response of EPIDs has been well characterized, in part through the development of computational models. Such models, however, have generally made simplifying assumptions with regards to the transport of optical photons within these detectors. The goal of this work was to develop and experimentally validate a new Monte Carlo (MC) model of an a-Si EPID that simulates both x-ray and optical photon transport in a self-contained manner. Using this model the authors establish a definitive characterization of the effects of optical transport on the dosimetric response of a-Si EPIDs employing gadolinium oxysulfide phosphor screens. The Geant4 MC toolkit was used to develop a model of an a-Si EPID that employs standard electromagnetic and optical physics classes. The sensitivity of EPID response to uncertainties in optical transport parameters was evaluated by investigating their effects on the EPID point spread function (PSF). An optical blur kernel was also calculated to isolate the component of the PSF resulting purely from optical transport. A 6 MV photon source model was developed and integrated into the MC model to investigate EPID dosimetric response. Field size output factors and relative dose profiles were calculated for a set of open fields by separately scoring energy deposited in the phosphor and optical absorption events in the photodiode. These were then compared to quantify effects resulting from optical photon transport. The EPID model was validated against experimental measurements taken using a research EPID. Optical photon scatter within the phosphor screen noticeably broadened the PSF. Variations in optical transport parameters reported in the literature caused fluctuations in the PSF full width at half maximum (FWHM) and full width at tenth maximum (FWTM) of less than 3% and 5%, respectively, confirming model robustness. Greater deviations (up to 9.5% and 36% for FWHM and FWTM, respectively) were observed when optical parameters were largely different from reference values. When scoring energy deposition in the phosphor, measured and calculated output factors agreed within statistical uncertainties and at least 94% of the MC simulated profile data points passed 3%/3 mm ?-index criterion for all field sizes considered. Despite statistical uncertainties in optical simulations arising from computational limitations, no differences were observed between optical and energy deposition profiles. Simulations demonstrated noticeable blurring of the EPID PSF when scoring optical absorption events in the photodiode relative to energy deposition in the phosphor. However, modeling the standard electromagnetic transport alone should suffice when using MC methods to predict EPID dose-response to static, open 6 MV fields with a standard a-Si photodiode array."
According to the news editors, the research concluded: "Therefore, using energy deposition in the phosphor as a surrogate for EPID dose-response is a valid approach that should not require additional corrections for optical transport effects in current a-Si EPIDs employing phosphor screens."
For more information on this research see: Characterization of optical transport effects on EPID dosimetry using Geant4. Medical Physics, 2013;40(4):041708. Medical Physics can be contacted at: Amer Assoc Physicists Medicine Amer Inst Physics, Ste 1 No 1, 2 Huntington Quadrangle, Melville, NY 11747-4502, USA. (American Association of Physicists in Medicine - www.aapm.org; Medical Physics - online.medphys.org/)
The news correspondents report that additional information may be obtained from S.J. Blake, Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW 2006, Australia. Additional authors for this research include P. Vial, L. Holloway, P.B. Greer, A.L. McNamara and Z. Kuncic (see also Health and Medicine).
The publisher's contact information for the journal Medical Physics is: Amer Assoc Physicists Medicine Amer Inst Physics, Ste 1 No 1, 2 Huntington Quadrangle, Melville, NY 11747-4502, USA.
Keywords for this news article include: Sydney, Radiotherapy, Health and Medicine, Australia and New Zealand.
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