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Initial evaluation of a commercial EPID modified to a novel direct-detection configuration for radiotherapy dosimetry

Journal Article


Abstract


  • Electronic portal imaging devices (EPIDs) integrated with medical linear accelerators utilize an indirect-detection EPID configuration (ID-EPID). Amorphous silicon ID-EPIDs provide high quality low dose images for verification of radiotherapy treatments but they have limitations as dosimeters. The standard ID-EPID configuration includes a high atomic number phosphor scintillator screen, a 1 mm copper layer, and other nonwater equivalent materials covering the detector. This configuration leads to marked differences in the response of an ID-EPID compared to standard radiotherapy dosimeters such as ion chambers in water. In this study the phosphor and copper were removed from a standard commercial EPID to modify the configuration to a direct-detection EPID (DD-EPID). Using solid water as the buildup and backscatter for the detector, dosimetric measurements were performed on the DD-EPID and compared to standard dose-in-water data for 6 and 18 MV photons. The sensitivity of the DD-EPID was approximately eight times less than the ID-EPID but the signal was sufficient to produce accurate and reproducible beam profile measurements for open beams and an intensity-modulated beam. Due to the lower signal levels it was found necessary to ensure that the dark field correction (no radiation) DD-EPID signal was stable or updated frequently. The linearity of dose response was comparable to the ID-EPID but with a greater under-response at low doses. DD-EPID measurements of field size output factors and beam profiles at the depth of maximum dose (dmax), and tissue-maximum ratios between the depths of 0.5 and 10 cm, were in close agreement with dose in water measurements. At depths beyond dmax the DD-EPID showed a greater change in response to field size than ionisation chamber measurements and the beam penumbrae were broader compared to diode scans. The modified DD-EPID configuration studied here has the potential to improve the performance of EPIDs for dose verification of radiotherapy treatments. © 2008 American Association of Physicists in Medicine.

UOW Authors


  •   Baldock, Clive (external author)

Publication Date


  • 2008

Citation


  • Vial, P., Greer, P. B., Oliver, L., & Baldock, C. (2008). Initial evaluation of a commercial EPID modified to a novel direct-detection configuration for radiotherapy dosimetry. Medical Physics, 35(10), 4362-4374. doi:10.1118/1.2975156

Scopus Eid


  • 2-s2.0-52949139395

Start Page


  • 4362

End Page


  • 4374

Volume


  • 35

Issue


  • 10

Abstract


  • Electronic portal imaging devices (EPIDs) integrated with medical linear accelerators utilize an indirect-detection EPID configuration (ID-EPID). Amorphous silicon ID-EPIDs provide high quality low dose images for verification of radiotherapy treatments but they have limitations as dosimeters. The standard ID-EPID configuration includes a high atomic number phosphor scintillator screen, a 1 mm copper layer, and other nonwater equivalent materials covering the detector. This configuration leads to marked differences in the response of an ID-EPID compared to standard radiotherapy dosimeters such as ion chambers in water. In this study the phosphor and copper were removed from a standard commercial EPID to modify the configuration to a direct-detection EPID (DD-EPID). Using solid water as the buildup and backscatter for the detector, dosimetric measurements were performed on the DD-EPID and compared to standard dose-in-water data for 6 and 18 MV photons. The sensitivity of the DD-EPID was approximately eight times less than the ID-EPID but the signal was sufficient to produce accurate and reproducible beam profile measurements for open beams and an intensity-modulated beam. Due to the lower signal levels it was found necessary to ensure that the dark field correction (no radiation) DD-EPID signal was stable or updated frequently. The linearity of dose response was comparable to the ID-EPID but with a greater under-response at low doses. DD-EPID measurements of field size output factors and beam profiles at the depth of maximum dose (dmax), and tissue-maximum ratios between the depths of 0.5 and 10 cm, were in close agreement with dose in water measurements. At depths beyond dmax the DD-EPID showed a greater change in response to field size than ionisation chamber measurements and the beam penumbrae were broader compared to diode scans. The modified DD-EPID configuration studied here has the potential to improve the performance of EPIDs for dose verification of radiotherapy treatments. © 2008 American Association of Physicists in Medicine.

UOW Authors


  •   Baldock, Clive (external author)

Publication Date


  • 2008

Citation


  • Vial, P., Greer, P. B., Oliver, L., & Baldock, C. (2008). Initial evaluation of a commercial EPID modified to a novel direct-detection configuration for radiotherapy dosimetry. Medical Physics, 35(10), 4362-4374. doi:10.1118/1.2975156

Scopus Eid


  • 2-s2.0-52949139395

Start Page


  • 4362

End Page


  • 4374

Volume


  • 35

Issue


  • 10