SU‐E‐T‐94: Surface Dose Characteristics of Flattened and Flattening Filter Free Megavoltage Photon Beam

Purpose: To evaluate the surface dose characteristics of the flattened and flattening filter free (FFF) megavoltage photon beam with radiochromic film and compare the Result: with that from ion chamber. Methods: A Varian TrueBeam linear accelerator capable to produce both flattened and FFF photon at 6 MV and 10 MV was used in this study. Surface dose measurement was performed with both Gafchromic EBT3 radiochromic film and PTW23342 parallel plate chamber. Build‐up dose was measured by adding different thickness of solid water slabs on the top of the measuring devices, at 100 cm SSD with field size of 2×2, 5×5, 10×10 and 20×20 cm2. The measurement data were normalized to PDD at 5 cm depth which was obtained from commissioning data. Percentage surface dose was calculated by extrapolating the PDD from 2 mm build‐up to the surface. Results: Surface doses of the FFF photon were 5 – 10% higher than that of the flattened photon at field size smaller than 10×10 cm2. In film measurement, the surface doses for 6X flattened and FFF photon increased from 12.0% to 30.6% and 20.5% to 36.1% respectively, when field size increased from 2×2 to 20×20 cm2; for 10X flattened and FFF photon, they were increased from 6.1% to 24.2% and 11.2% to 23.5% respectively. Surface dose obtained in chamber measurement showed 2 – 6% higher than the corresponding measurement with film, but with a similar trend with that in the film measurement. Conclusion: FFF photon has a higher surface dose than flattened photon at field size smaller than 10×10 cm2. Surface dose measured from chamber is found to be higher than that from film, which is probably due to the chamber design. Nonetheless, the trend of surface dose increases with field size is consistent between the two measuring devices.