This study presents the basic dosimetric properties of photon beams of a Versa HD linear accelerator (linac), which is capable of delivering flattening filter‐free (FFF) beams with a beam quality equivalent to the corresponding flattened beams based on comprehensive beam data measurement. The analyzed data included the PDDs, profiles, penumbra, out‐of‐field doses, surface doses, output factors, head and phantom scatter factors, and MLC transmissions for both FFF and flattened beams of 6 MV and 10 MV energy from an Elekta Versa HD linac. The 6MVFFF and 10MVFFF beams had an equivalent mean energy to the flattened beams and showed less PDD variations with the field sizes. Compared with their corresponding flattened beams, Dmax was deeper for FFF beams for all field sizes; the ionization ratio variations with the field size were lower for FFF beams; the out‐of‐field doses were lower and the penumbras were sharper for the FFF beams; the off‐axis profile variations with the depths were lesser for the FFF beams. Further, the 6MVFFF and 10MVFFF beams had 35.7% and 40.9% less variations in output factor with the field size, respectively. The collimator exchange effect was reduced in the FFF mode. The head scatter factor showed 59.1% and 73.6% less variations, on average, for the 6MVFFF and 10MVFFF beams, respectively; the variations in the phantom scatter factor were also smaller. The surface doses for all beams increased linearly with the field size. The 6MVFFF and 10MVFFF beams had higher surface doses than the corresponding flattened beams for field sizes of up to 10×10cm2 but had lower surface doses for larger fields. Both FFF beams had lower average MLC transmissions than the flattened beams. The finding that the FFF beams were of equivalent quality to the corresponding flattened beams indicates a significant difference from the data on unmatched FFF beams.PACS number(s): 87.56.bd, 87.55.Qr
Purpose:To provide data on the relative surface dose of photon beams of Versa HD linear accelerator designed to delivered the Flattening Filter Free (FFF) beam having beam quality equivalent to the corresponding Flattened beam.This study involves the comparative evaluation of the entrance dose of FFF and Flattened photon beams in the buildup region.Methods:Surface dose was measured using Markus parallel plate chamber with the thin flat entrance window thickness of 0.025mm of water equivalent material in solid water phantom. Data were collected for square fields of 2×2cm2, 3×3cm2,4×4cm2,5×5cm2,7×7cm2,10×10cm2 and 20×20cm2 created in Monaco Treatment Planning System.The surface dose was define for this study as the relative dose at depth of 0.5mm with respect to the dose at dmax.The same Markus chamber was used to investigate the dose in the buildup region for all the square fields.The buildup depths for 6MV flattened and FFF beams were 0 to 18mm and for 10MV Flattened and FFF beams were 0 to 24mm. All values were determined in relation to the depth at dose maximum dmax.Results:The relation of surface dose for both Flattened and FFF beams with field sizes were linear. The surface dose for smaller field size upto 10×10cm2 for both FFF beams were higher than Flattened beams but the variation was marginal. The difference was 0.3 percentage at 6MV and 1.9 percentage at 10MV for a 10×10cm2 field but for the larger field the relative surface dose was lower for FFF beam with difference of 3.6 percentage at 6MV and 4.3 percentage at 10MV for a 20×20cm2 field.Conclusion:The equivalent quality FFF Photons produce higher surface dose for smaller field sizes upto 10×10cm2 but lower surface dose for large field sizes than the conventional flattened beam. The percentage variation was marginal for smaller field sizes and may not be clinically significant.
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