Reduction of superficial radiation dose with bolus in passive scattering proton beam therapy

PURPOSE: In passive scattering proton beam therapy, scattered protons from the snout and aperture increase the superficial dose, however, treatment planning systems (TPSs) based on analytic algorithms (such as proton convolution superposition) are often inaccurate in this aspect. This additional dose can cause permanent alopecia or severe radiation dermatitis. This study aimed to evaluate the effect of bolus on the superficial radiation dose in passive scattering proton beam therapy. METHODS: We drew a clinical target volume (CTV) and a scalp-p (phantom), and created plans using a TPS for a solid water phantom with and without bolus. We calculated the dose distribution in the established plans independently with Monte Carlo (MC) simulation and measured the actual dose distribution with an array of ion chambers and radiochromic films. To assess the clinical impact of bolus on scalp dose, we conducted independent dose verification using MC simulation in a clinical case. RESULTS: In the solid water phantom without bolus, the calculated scalp-p volume receiving 190 cGy was 20% with TPS but 80% with MC simulation when the CTV received 200 cGy. With 2 cm bolus, this decreased from 80% to 10% in MC simulation. With the measurements, average superficial dose to the scalp-p was reduced by 5.2% when 2 cm bolus was applied. In the clinical case, the scalp-c (clinical) volume receiving 3000 cGy decreased from 74% to 63% when 2 cm bolus was applied. CONCLUSION: This study revealed that bolus can reduce radiation dose at the superficial body area and alleviate toxicity in passive scattering proton beam therapy.