Effect of various physical parameters on surface and build-up dose for 15-MV X-rays.

The purpose of this study was to find out the effect of various physical parameters on the skin and build-up doses of 15-MV photon beams. The effects of field dimensions, acrylic shadow tray, focus to-skin distance (FSD) on surface and buildup dose were determined for open, motorized 60° wedge (MW) and blocked fields. A 'Markus' plane parallel plate chamber was used for these measurements in an Elekta (6-15MV) linear accelerator. The surface dose for MW fields was lower than the dose for an open field, but the trend reversed for large fields and higher degree wedges. With the use of an acrylic shadow 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. The percentage depth dose of 10 × 10 cm(2) field at surface (PDD(0)) for open beam were 13.89%, 11.71%, and 10.74% at 80 cm, 100 cm, and 120 cm FSD, respectively. The blocking tray increased PDD(0) of 10 × 10 cm(2) field to 26.29%, 14.01%, and 11.53%, while the motorized 60° wedge decreased PDD(0) to 11.32%, 9.7%, and 8.9 % at these FSDs. The maximum PDD difference seen at surface (i.e., skin) for 5 × 5 cm(2), 15 × 15 cm(2), and 30 × 30 cm(2) are 0.5%, 4.6%, and 5.6% for open field and 0.9%, 4.7%, and 7.2% for motorized 60° wedge field, when FSDs varied from 80 cm to 120 cm. The maximum PDD difference seen at surface for 5 × 5 cm(2), 15 × 15 cm(2), and 30 × 30 cm(2) fields are 5.6%, 22.8%, and 29.6%, respectively, for a 1.0-cm perspex-blocking tray as the FSD is changed. The maximum PDD difference was seen at the surface (i.e., skin) and this decreased with increasing depth.