SU‐E‐T‐161: Small Field Surface Doses for 6X and 10X Photon Beams ‐ FFF versus FF

Purpose: To measure and compare small field surface doses for Flattening filter free (FFF) and flattened (FF) photon beams. Methods: Measurements were performed for photon beams of two nominal energies (6 and 10 MV) of a Varian Linac (TrueBeamSTX). TrueBeam delivers 6 MV and 10 MV photon beams with both ‐ a flattening filter in place (FF) or using an open port where a thin foil is used instead of a flattening filter (FFF). We refer to the four photon beams investigated to as 6XFF, 6XFFF, 10XFF, and 10XFFF. All measurements were performed in IBA Blue Water Phantom (IBA‐Dosimetry) and a SFD diode (IBA, thickness of measuring volume 60 m) was employed to measure surface doses. Surface doses were expressed relative to the dose at dose maximum for the respective energy. Field sizes were varied from 1×1 cm2 to 10×10 cm2. Results: Measurements in the 1×1 cm2 to 10×10 cm2 field size range resulted in higher surface doses for FFF beams compared to FF beams. Percentage surface dose increases for FFF beams compared to FF beams were — for a 1× 1 cm2 increases were 36% and 60% for 6X and 10X beams respectively compared to corresponding 31.5 and 50.0% increases for a 5×5 cm2 field size. Overall, surface doses relative to dmax ranged from 0.37 – 0.44 and 0.21 – 0.28 for field sizes of 1×1 cm2 to 10×10 cm2 for the 6XFF and 10XFF beams respectively. The corresponding range of values for the 6XFFF and 10XFFF beams were 0.50 – 0.54 and 0.34 – 0.37 respectively. Conclusions: FFF beam change the dosimetric characteristics of photon beams by softening the energy spectra compared to beam hardened FF beams, thus changing surface doses. Our study evaluates the magnitude of these changes for small field sizes. The results confirm literature observations.