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EPID dosimetry: Effect of different layers of materials on absorbed dose response

Gustafsson, Helen LU ; Vial, Philip; Kuncic, Zdenka; Baldock, Clive and Greer, Peter B. (2009) In Medical Physics 36(12). p.5665-5674
Abstract
Purpose: Commercial EPIDs are normally used in indirect detection mode (iEPID) where incident x-ray photons are converted to optical photons in a phosphor scintillator, which are then detected by a photodiode array. The EPIDs are constructed from a number of nonwater equivalent materials which affect the dose response of the detector. The so-called direct detection EPIDs (dEPIDs), operating without the phosphor layer, have been reported to display dose response close to in-water data. In this study, the effect that different layers of materials in the EPID have on the dose response was experimentally investigated and evaluated with respect to changes in field size response and beam profiles. Methods: An iEPID was disassembled and the... (More)
Purpose: Commercial EPIDs are normally used in indirect detection mode (iEPID) where incident x-ray photons are converted to optical photons in a phosphor scintillator, which are then detected by a photodiode array. The EPIDs are constructed from a number of nonwater equivalent materials which affect the dose response of the detector. The so-called direct detection EPIDs (dEPIDs), operating without the phosphor layer, have been reported to display dose response close to in-water data. In this study, the effect that different layers of materials in the EPID have on the dose response was experimentally investigated and evaluated with respect to changes in field size response and beam profiles. Methods: An iEPID was disassembled and the different layers of materials were removed or replaced with other materials. Data were also obtained on and off the support arm and with a sheet of opaque paper blocking the optical photons from the gadolinium oxysulfide (Gd2S2O: Tb) phosphor layer. Field size response was measured for field sizes ranging from 2 x 2 to 25 x 25 cm(2), and profiles for the 25 x 25 cm(2) beams were extracted from the data. Results: The iEPID configuration was found to be very sensitive to backscatter. The increases in output with solid water backscatter compared to the no backscatter case were 14.7% and 6.6% at the largest field size investigated for the 6 and 18 MV beams, respectively. The Gd2S2O: Tb phosphor layer had a large influence on field size response as well as beam profiles for 6 MV photons, while no major effects were observed for the 18 MV beam. For 18 MV large differences in dose response were found when the standard 1 mm Cu buildup was changed for d(max) equivalent Cu or solid water buildup, indicating that head scatter largely influences dose response for this energy. When the optical photons originating in the Gd2S2O: Tb layer were blocked from reaching the photodiodes, both field size output data and beam profiles corresponded well with data obtained in the dEPID configuration as well as reference ion chamber data for both energies. Conclusions: As expected, changing the layers of material in the EPID had a dramatic effect on dose response, which was often quite complex. For 6 MV, the complex dose response is mainly caused by the optical photons from the Gd2S2O: Tb layer, while insufficient filtering of scattered radiation largely affects the dose response for the 18 MV beam. The iEPID was also found to be very sensitive to backscatter for both energies. Blocking the optical photons created in the Gd2S2O: Tb layer essentially changed the iEPID configuration into the dEPID configuration, thus demonstrating great potential for a system that can be optimized for both imaging and dosimetry. (C) 2009 American Association of Physicists in Medicine. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
configuration, electronic portal imaging device, dosimetry, profiles, field size response, response, dose
in
Medical Physics
volume
36
issue
12
pages
5665 - 5674
publisher
American Association of Physicists in Medicine
external identifiers
  • wos:000272157500031
  • scopus:72049110766
ISSN
0094-2405
DOI
10.1118/1.3245886
language
English
LU publication?
yes
id
e4264a07-a823-4f2f-8df5-6b4d7804ba7b (old id 1517645)
date added to LUP
2010-01-04 16:06:53
date last changed
2017-07-09 04:03:59
@article{e4264a07-a823-4f2f-8df5-6b4d7804ba7b,
  abstract     = {Purpose: Commercial EPIDs are normally used in indirect detection mode (iEPID) where incident x-ray photons are converted to optical photons in a phosphor scintillator, which are then detected by a photodiode array. The EPIDs are constructed from a number of nonwater equivalent materials which affect the dose response of the detector. The so-called direct detection EPIDs (dEPIDs), operating without the phosphor layer, have been reported to display dose response close to in-water data. In this study, the effect that different layers of materials in the EPID have on the dose response was experimentally investigated and evaluated with respect to changes in field size response and beam profiles. Methods: An iEPID was disassembled and the different layers of materials were removed or replaced with other materials. Data were also obtained on and off the support arm and with a sheet of opaque paper blocking the optical photons from the gadolinium oxysulfide (Gd2S2O: Tb) phosphor layer. Field size response was measured for field sizes ranging from 2 x 2 to 25 x 25 cm(2), and profiles for the 25 x 25 cm(2) beams were extracted from the data. Results: The iEPID configuration was found to be very sensitive to backscatter. The increases in output with solid water backscatter compared to the no backscatter case were 14.7% and 6.6% at the largest field size investigated for the 6 and 18 MV beams, respectively. The Gd2S2O: Tb phosphor layer had a large influence on field size response as well as beam profiles for 6 MV photons, while no major effects were observed for the 18 MV beam. For 18 MV large differences in dose response were found when the standard 1 mm Cu buildup was changed for d(max) equivalent Cu or solid water buildup, indicating that head scatter largely influences dose response for this energy. When the optical photons originating in the Gd2S2O: Tb layer were blocked from reaching the photodiodes, both field size output data and beam profiles corresponded well with data obtained in the dEPID configuration as well as reference ion chamber data for both energies. Conclusions: As expected, changing the layers of material in the EPID had a dramatic effect on dose response, which was often quite complex. For 6 MV, the complex dose response is mainly caused by the optical photons from the Gd2S2O: Tb layer, while insufficient filtering of scattered radiation largely affects the dose response for the 18 MV beam. The iEPID was also found to be very sensitive to backscatter for both energies. Blocking the optical photons created in the Gd2S2O: Tb layer essentially changed the iEPID configuration into the dEPID configuration, thus demonstrating great potential for a system that can be optimized for both imaging and dosimetry. (C) 2009 American Association of Physicists in Medicine.},
  author       = {Gustafsson, Helen and Vial, Philip and Kuncic, Zdenka and Baldock, Clive and Greer, Peter B.},
  issn         = {0094-2405},
  keyword      = {configuration,electronic portal imaging device,dosimetry,profiles,field size response,response,dose},
  language     = {eng},
  number       = {12},
  pages        = {5665--5674},
  publisher    = {American Association of Physicists in Medicine},
  series       = {Medical Physics},
  title        = {EPID dosimetry: Effect of different layers of materials on absorbed dose response},
  url          = {http://dx.doi.org/10.1118/1.3245886},
  volume       = {36},
  year         = {2009},
}