SU‐E‐T‐111: Radiochromic Film Measurements of Small Field Surface Doses from Flattened and Unflattened 6MV and 10MV Beams

Purpose: To measure surface doses from small fields from flattened and unflattened 6MV and 10MV photon beams using radiochromic film.Methods: GAFCHROMIC EBT2 film was used in a solid water phantom to make surface dose measurements from photon beams generated by a Varian TrueBeam Stx linac. Four energies were included: 6MV and 10MV, as well as two corresponding flattening filter free beams (6FFF and 10FFF). The field sizes considered where 10, 5, 4, 3, 2, 1 cm2. One 3×3 cm piece of EBT2 film was irradiated at the surface and at dmax in a solid water phantom for every energy and field size. Films were also placed at dmax and irradiated to obtain a calibration curve. All films were scanned in an EPSON V700 flatbed scanner and analyzed according to established protocols using in‐house developed software. Results: The measurements of surface dose for each field were normalized by the dose measured at dmax. Surface doses ranged from 10% for a 1 cm2 6MV field to 25% for a 10 cm2 6MV field. The ranges for other energies were 16–33%, 5–17%, and 10– 17% for 6FFF, 10MV, and 10FFF, respectively. As expected, the surface dose for both 6MV and 6FFF fields are higher than surface doses from 10MV and 10FFF fields. The surface dose relative to dmax is shown to be a monotonically increasing function of field size. Unflattened beams exhibit a higher surface dose in comparison to flattened beams and this discrepancy is larger for 6MV and 6FFF fields Conclusions: This work makes use of a simple technique for measuring surface doses from small fields relevant to SRS and SBRT, taking advantage of the high resolution offered by radiochromic film. The surface dose measured for a range of standard and small field sizes for flattened and unflattened beams has been presented.