A novel tool for predicting the dose distribution of non-sealed 188Re (Rhenium) resin in non-melanoma skin cancers (NMSC) patients

BackgroundHigh-dose rate brachytherapy using a non-sealed (188)Rhenium resin (Re-188) 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-188-based resin over a plastic foil placed on the target skin surface. However, there is no treatment planning tool to assess the Re-188 activity needed for a personalized treatment. PurposeThe paper aims to present a novel Monte Carlo (MC)-based tool for Re-188-based resin activity and dose calculation, experimentally validated using Gafchromic EBT3 films. MethodsMC simulations were carried out using FLUKA modeling density and composition of Re-188 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-188 eluate and a heterogeneous activity distribution of Re-188 resin mimic the patient treatment. ResultsThe 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(2), 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-188. Worst agreements were observed for absorbed doses lower than 0.3 Gy. ConclusionsOur results support the implementation of our MC-based tool in the practical routine for calculating the Re-188 resin activity and treatment parameters necessary for obtaining the prescribed minimal target dose.

Automated summary

This paper presents a novel Monte Carlo-based tool for calculating the activity and dose of Re-188-based resin. The tool allows personalized calculation of treatment parameters and has been experimentally validated. The results demonstrate the accuracy of the tool in calculating the activity distribution of Re-188 resin.