Time-Resolved EPID Dosimetry for Validating ArcIMRT
I Yeo1*, J Jung2, J Kim3, B Patyal1, (1) Loma Linda Univ Medical Center, Loma Linda, CA, (2) East Carolina Univ, Greenville, NC, (3) University of Pittsburgh Cancer Institute, Pittsburgh, PASU-E-T-366 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
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.