AbstractID: 8258 Title: Buildup Region Dependence on Photon Dose Delivery Technique for IMRT
The more complex delivery techniques required for implementation of IMRT based on inverse planning optimization have changed
the relationship between dose at depth and dose at the surface buildup regions. Surface dose is dependent on electron contamination
primarily from the flattening filter and secondarily from air and other collimation systems. The electron contamination reaching the
tissue surface depends on the source of the contamination and the MLC position and/or motion. Measurements were performed in
water and plastic water simulant using a parallel plate chamber, TLD, and GaFchromic film and Kodak XV2 film at depths of 2, 5, 15
mm, and 10 cm for a 6 MV X-ray beam. Data were normalized to the 10 cm value of each field. To better understand the effect,
measurements were performed with 10x10-cm2 fields which were delivered with 10 static 1x10-cm2 strips, 5 static 2x10-cm2 strips, 3
static 3.5 or 3x10-cm2 strips, and 1x10-cm2 sliding window, then compared with the a static 10x10-cm2 open field. The % depth doses
at 2 mm depth (normalized at 10 cm depth) were lower by 11%, 7%, 6%, and 10%, respectively, compared to the open field. These
differences are most likely due to an increased MLC transmission which is more penetrating. Example DMLC and SMLC fields were
also studied to assess the variation due to a delivery technique. The overall effect on the surface dose is dependent on the multileaf
delivery system for IMRT. Supported in part by NIH grant P01-CA59827.