A novel tool for predicting the dose distribution of non-sealed Re (Rhenium) resin in non-melanoma skin cancers (NMSC) patients.

Background: High-dose rate brachytherapy using a non-sealed Rhenium resin ( Re) is a recently approved treatment option for non-melanoma skin cancer (NMSC). The treatment goal is to deliver a personalized absorbed dose to the deepest point of neoplastic infiltration corresponding to the minimal target dose. The treatment consists of the application of a Re-based resin over a plastic foil placed on the target skin surface. However, there is no treatment planning tool to assess the Re activity needed for a personalized treatment.

Purpose: The paper aims to present a novel Monte Carlo (MC)-based tool for Re-based resin activity and dose calculation, experimentally validated using Gafchromic EBT3 films.

Methods: MC simulations were carried out using FLUKA modeling density and composition of Re resin. The MC-based look up table (LUT) was incorporated in an ad hoc developed tool. The proposed tool allows the personalized calculation of treatment parameters (i.e., activity to be dispensed, the treatment duration, and dose volume histograms), according to the target dimension. The proposed tool was compared using Bland-Altman analysis to the previous calculation approaches conducted using VARSKIN in a retrospective cohort of 76 patients. The tool was validated in ad hoc experimental set ups using a stack of calibrated Gafchromic EBT3 films covered by a plastic film and exposed using a homogenous activity distribution of Re eluate and a heterogeneous activity distribution of Re resin mimic the patient treatment.

Results: The agreement between the proposed tool and VARSKIN was evaluated on the investigated cohort with median range of target area, target depth, and treatment time equal to 4.8 [1.0-60.1] cm , 1.1 [0.2-3.0] mm, and 70 [21-285] min, with a median range of target dose (Gy) of 23.5 [10-54.9]. The calculated minimal target doses, ranged from 1% to 10% for intermediate target depths (1.2 ± 0.7 mm), while showing significant differences in the estimation of superficial (maximal) target doses. The agreement between MC calculation and measurements at different plans in a stack of Gafchromic EBT3 films was within 10% for both the homogenous and heterogeneous activity distribution of Re. Worst agreements were observed for absorbed doses lower than 0.3 Gy.

Conclusions: Our results support the implementation of our MC-based tool in the practical routine for calculating the Re resin activity and treatment parameters necessary for obtaining the prescribed minimal target dose.