2-D radiation therapy dosimetry using EPIDs: Dose response variation between 3 siemens electronic portal imaging devices (EPIDs)
Prerequisite for any new clinical dosimeter is a detailed understanding of the detector’s dose response behavior. The purpose of the present study was to investigate the level of variability in dose response characteristics of 3 siemens EPIDs for both 6 and 18 MV photons. Dose response tests for linearity, field size, short- and long-term reproducibilities, ghosting effects and dose rate dependence were undertaken for three different model Siemens EPIDs on three dosimetrically matched linear accelerators. All three EPIDs showed a linear dose response above 20 monitor units. The pixel sensitivity for EPID2 and EPID3 agreed to within 0.5%. EPID1 was 6.3% (6 MV) and 2.3% (18 MV) greater than for EPID2 and EPID3. The field size response and dose rate response agreed within 1% for all three EPIDs. The short-term and long-term reproducibilities for all EPIDs were within 0.5% and 1% respectively. The maximum increase in relative response due to the ghosting effect was 0.5%. The off-axis profiles from uncorrected gain files agreed to within 2% for EPID2 and EPID3 at 6 MV and 18 MV respectively. The off-axis profiles for EPID1 had more pronounced horns. The different dose response behavior of EPID1 is due to a thicker phosphor scintillator compared to EPID2 and EPID3. EPID1 was significantly more sensitive to dose and energy variations and would require separate calibration data for EPID dosimetry. EPIDs with the same phosphor had no significant difference in dose response. Differences in EPID/phosphor design must be considered when commissioning EPIDs for clinical dosimetry.