Analyses of Surface Dose from High Energy Photon Beams for Different Clinical Setup Parameters


Abstract: Surface and build-up region doses of 6, 15 and 25 MV photon beams were investigated for several clinical setup parameters. Evaluated setups included open fields, physical wedge (PW) fields, virtual wedge (VW) fields and blocked fields. The effects of field size, acrylic block tray, source-to-surface distance (SSD) and off-axis distance on surface dose were determined for each of these setups. Siemens Mevatron MD2 (6-15 MV) and Mevatron-KD2 (6-25 MV) linear accelerators were used to measure both the surface and build-up region doses. The surface dose increased as field size increased in open fields. There was no significant difference between 6 MV photon beam surface dose values for both machines. The surface dose for PW fields was lower than the dose for an open field, but higher in the case of large fields and higher degree wedges. The VW field surface doses were higher than the dose for PW and quite similar to the open fields. With the use of an acrylic block tray, the surface dose increased for all field sizes, but the increase was dominant for large fields. The surface dose for blocked fields was lower than the dose for open fields. As SSD decreased, the surface dose increased, and this effect was dominant, especially in larger field sizes with an acrylic block tray.

Keywords: skin dose, wedge, blocked field

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