Surface buildup dose dependence on photon field delivery technique for IMRT

The more complex delivery techniques required for implementation of intensity-modulated radiotherapy (IMRT) based on inverse planning optimization have changed the relationship between dose at depth and dose at buildup regions near the surface. Surface buildup dose is dependent on electron contamination primarily from the unblocked view of the flattening filter and secondarily from air and collimation systems. To evaluate the impact of beam segmentation on buildup dose, measurements were performed with 10 x 10 cm2 fields, which were delivered with 3 static 3.5 x 10 cm2 or 3 x 10 cm2 strips, 5 static 2 x 10 cm2 strips, 10 static 1 x 10 cm2 strips, and 1.1 x 10 cm2 dynamic delivery, compared with a 10 x 10 cm2 open field. Measurements were performed in water and Solid Water using parallel plate chambers, a stereotactic diode, and thermoluminescent dosimeters (TLDs) for a 6 MV X-ray beam. Depth doses at 2 mm depth (relative to dose at 10 cm depth) were lower by 6%, 7%, 11%, and 10% for the above field delivery techniques, respectively, compared to the open field. These differences are most influenced by differences in multileaf collimator (MLC) transmission contributing to the useful beam. An example IMRT field was also studied to assess variations due to delivery technique (static vs. dynamic) and intensity level. Buildup dose is weakly dependent on the multileaf delivery technique for efficient IMRT fields.