Technical note: Evaluation of a silicone-based custom bolus for radiation therapy of a superficial pelvic tumor

Abstract

Purpose: Use of standard-of-care radiation therapy boluses may result in air-gaps between the target surface and bolus, as they may not adequately conform to each patient's unique topography. Such air-gaps can be particularly problematic in cases of superficial pelvic tumor radiation, as the density variation may result in the radiation delivered to the target site being inconsistent with the prescribed dose. To increase bolus fit and thereby dose predictability and homogeneity, we designed and produced a custom silicone bolus for evaluation against the clinical standard. Methods: A custom bolus was created for the pelvic regions of both an anthropomorphic phantom and a pelvic patient with squamous cell carcinoma of the penile shaft. Molds were designed using computed tomography (CT) scans, then 3D-printed and cast with silicone rubber to yield the boluses. Air-gap measurements were performed on custom and standard-of-care Superflab gel sheet boluses by analyzing total volume between the bolus and target surface, as measured from CT scans. Therapeutic doses of radiation were delivered to both boluses. Radiation dose was measured and compared to the expected dose using nine optically stimulated luminescent dosimeters (OSLDs) placed on the phantom. Results: Mean air-gap volume between the bolus and phantom was decreased from 314 ± 141 cm3 with the standard bolus to 4.56 ± 1.59 cm3 using the custom device. In the case of the on-treatment patient, air-gap volume was reduced from 169 cm3 with the standard bolus to 46.1 cm3 with the custom. Dosimetry testing revealed that the mean absolute difference between expected and received doses was 5.69%±4.56% (15.1% maximum) for the standard bolus and 1.91%±1.31% (3.51% maximum) for the custom device. Areas of greater dose difference corresponded to areas of larger air-gap. Conclusions: The custom bolus reduced air-gap and increased predictability of radiation dose delivered compared to the standard bolus. The custom bolus could increase the certainty of prescribed dose-delivery of radiation therapy for superficial tumors.