SU-E-T-220: Investigation of Intrafraction Changes to Photon Beam Profiles

Purpose: Treatment Planning Systems model beam profiles to be static and independent of gantry angle. This work investigates how beam profiles change over time, with varying gantry angle. Methods: 200 cGy was delivered to ICProfiler (Sun Nuclear) on 2 Elekta Synergys and 1 Varian 21EX linacs at 8 gantry angles and 1 clockwise arc. Measurements were recorded 8 times per second. To investigate the initial portion of the beams, single-shot and multi-control-point beams were measured. The differences in beam profiles were analyzed as well as the length of time for the beam to stabilize. Symmetry was calculated 5cm from the central axis at 100cm SSD. Field sizes ranged from 30×30 cm² to 40×40 cm². Results: The beam profiles changed over time and were dependent on gantry angle. The composite profile for each beam met 1% tolerance described in TG-142. It took up to 14.7, 3.7 and 11.3 cGy for the two Elektas and Varian respectively to satisfy the tolerance. All linacs had the most flat, symmetric, and stable beam profile with the gantry at 0 degrees, where the symmetry was 100.5%±0.55, 101.0%±0.52 and 101.1%±0.44 respectively. The symmetry was 100.7%±1.06, 101.1%±1.05 and 101.2%±0.64 at the gantry angle with the biggest discrepancy (90, 180 and 180 degrees respectively). Symmetry was better in the X direction for the Elektas and the Y for the Varian. Fluctuations in beam profiles corresponded to changes in dose rate. Conclusion: Flatness and symmetry changed over time and with gantry angle. Because Treatment Planning Systems do not account for these changes, it may be necessary to limit certain beams from clinical use, for example, beams with low MU. It may also be necessary to add additional QA procedures beyond those suggested in TG-142 to account for these changes