SU‐E‐T‐366: Time‐Resolved EPID Dosimetry for Validating ArcIMRT

Abstract

Purpose: The purpose of this study is to demonstrate time‐resolved dosimetry for verifying arc intensity modulated radiation therapy (arcIMRT) delivery, utilizing the continuous image acquisition of an electronic portal imaging device (EPID). Methods: An arcIMRT field was made of sliding, multi‐leaf collimator (MLC) motion of X1 leaves traveling from −5 cm to +5 cm and back, while Y opening was kept at 10 cm. For this field, this travel was repeated three times during the gantry rotation of 180°. The images were continuously acquired while the EPID was irradiated with 240 MU at a constant dose rate of 300 MU/min. By summing 10 frames, and thus reducing the temporal resolution to 1 second, a longitudinal non‐uniformity in the images was reduced to less than 3%. Dose images in EPID were also calculated by the XVMC code for MLC positions at 1 mm interval using the EPID model developed in our previous study. The calculated images at 1 sec resolution were then correlated to and compared with the images, validating the time‐resolved dosimetry. Results: Over the period of 48 seconds, 481 EPID images were acquired. The MLCs traveled at 1.25 cm/sec. Thus, 10 images were assigned to 1.25 cm of MLC travel for the correlation and for dose evaluation. For all gantry angles, the agreement of the gamma tests was above 90%, given 3 mm distance to agreement and 3% dose difference, when MLCs moved forwardly to the X2 side. However, when they travel back, the pass rates were below 90% due to MLC lagging which was detected in time‐resolved EPID dose imaging. Conclusion: Time‐resolved, four‐dimensional dose validation of arcIMRT was demonstrated, showing the temporal information of dynamic radiation delivery. This can be used for the validation of 4D treatment delivery techniques.