Dosimetric properties and clinical application of an a-Si EPID for dynamic IMRT quality assurance

Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for verification of intensity-modulated radiation therapy (IMRT) were investigated as a replacement for conventional verification tools. The portal dosimetry system of Varian's EPID (aS1000) has an integrated image mode for portal dosimetry (PD). The source-to-imager distance was 105 cm, and there were no extra buildup materials on the surface of the EPID in this study. Several dosimetric properties were examined. For clinical dosimetry, the dose distributions of dynamic IMRT beams for prostate cancer (19 patients, 97 beams) were measured by EPID and compared with the results of ionization chamber (IC) measurements. In addition, pretreatment measurements for prostate IMRT (50 patients, 309 beams) were performed by EPID and were evaluated by the gamma method (criterion: 3 mm/3 %). The signal-to-monitor unit ratio of PD showed dose dependence, indicating ghosting effects. Tongue-and-groove effects were observed as a result of the dose difference in the measured EPID images. The results of PD for clinical IMRT beams were in good agreement with the predicted dose image with average values of 1.37 and 0.25 for γ (max) and γ (ave), respectively. The point doses of PD were slightly, but significantly, higher than the results of IC measurements (p < 0.05 paired t test). However, this small difference seems clinically acceptable. This portal dosimetry system is useful as a rapid and convenient verification tool for dynamic IMRT.