We evaluated the effect of scatter on a build-up region based on the measured percent depth dose (PDD) of high-energy photon beams that penetrated a handmade build-up modifier (BM) as a substitute of bolus. BM scatter factors (S_BM) were calculated based on the PDDs of photon beams that penetrated through the BM. The calculated S_BM values were normalized to 1 at the square field side (SFS) of 30 mm without a BM. For the largest SFS (200 mm), the S_BM values for a 6-MV beam were 1.331, 1.519, 1.598, 1.641, and 1.657 for the corresponding BM thickness values. For a 10-MV beam, the S_BM values were 1.384, 1.662, 1.825, 1.913, and 2.001 for the corresponding BM thickness values. The BM yielded 76% of the bolus efficiency. We expect BM to become useful devices for deep-set patient body parts to which it is difficult to apply a bolus.

KeywordsBuild-up modifier (BM), BM scatter factor (S_BM), Percent depth dose (PDD), Surface dose