Modeling scatter through sides of island blocks used for intensity-modulated bolus electron conformal therapy.

Purpose: Passive Radiotherapy Intensity Modulators for Electrons (PRIME) devices are comprised of cylindrical tungsten island blocks imbedded in a machinable foam slab within the patient's cutout. Intensity-modulated bolus electron conformal therapy (IM-BECT) uses PRIME devices to reduce dose heterogeneity caused by the irregular bolus surface. Heretofore, IM-BECT dose calculations used the pencil beam redefinition algorithm (PBRA) assuming perfect collimation. This study investigates modeling electron scatter into and out the sides of island blocks.

Methods: Dose distributions were measured in a water phantom at 7, 13, and 20 MeV for devices having nominal intensity reduction factors of 1.000 (foam only), 0.937, 0.812, and 0.688, corresponding to nominal island block diameters (d ) of 0.158, 0.273, and 0.352 cm, respectively. Pencil beam theory derived an effective diameter (d ) to account for in-scattered electrons as a function of d and beam energy (E ). However, for out-scattered electrons, an effective diameter (d ) was estimated by best fitting measured data.

Results: In the modulated region (under island blocks, depth < R ), modified PBRA-calculated dose distributions showed 2%/2 mm passing rates for d  = 0.158, 0.273, and 0.352 cm of (100%, 100%, 100%) at 7 MeV, (100%, 100%, 93.5%) at 13 MeV, and (99.8%, 85.4%, and 71.5%) at 20 MeV. The largest dose differences (≤ 6%) occurred at the highest energy (20 MeV), largest d , shallowest depths (≤ 2 cm), and on central axis.

Conclusions: An equation for modeling island block scatter, d (d , E ), has been developed for use in the PBRA, insignificantly impacting calculation time. Although inaccuracy sometimes exceeded our 2%/2 mm criteria, it could be clinically acceptable, as superficial dose differences often fall inside the bolus. Also, patient PRIME devices are expected to have fewer large diameter island blocks than did test devices. Inaccuracies are attributed to out-scattered electrons having energy spectra different than the primary beams.