Abstract
Introduction The aim of our work was to study the influence of lung/soft-tissue (L/ST) interfaces on dosimetry in Targeted Radionuclide Therapy (TRT) by fast convolution of Voxel-S-Values (VSV). Materials and methods Two simple anthropomorphic phantoms have been simulated with the Monte Carlo software tool MCNPX in order to calculate the dose deposition of 131I, 177Lu and 90Y. Phantom 1 was a lung composed of lung tissue (LT, 0.26 g cm −3 ) surrounded by soft tissue (ST, 1.04 g cm −3 ). Phantom 2 was Phantom 1 plus a tumour of variable diameter (1, 2 and 4 cm). For each phantom, two simulations have been computed, the first one takes into account L/ST interfaces (heterogeneous simulation – HT) and the other one doesn't (homogeneous simulation – HM), as it's computed when using VSV fast convolution. Dose distributions have been compared in terms of dose ratio between HM and HT (RHM/HT = DHM/ DHT) at the organ (ORHM/HT) and voxel levels (VRHM/HT). Results In phantom 1, ORHM/HT for 90Y, 131I and 177Lu were respectively 0.989, 0.992 and 0.998 and mean values for VRHM/HT were 0.987, 0.996 and 0.999. Profiles on VRHM/HT show very few perturbations inside the lung (RHM/HT ≈ 1.00) and at interfaces (0.99 6 RHM/HT 6 1.01). In phantom 2, ORHM/HT for the three tumour sizes was 1.00 for 177Lu, 1.01 6 ORHM/HT 6 1.02 for 131I and 1.03 6 ORHM/HT 6 1.24 for 90Y. The highest values are found for the 1 cm tumour. At the voxel level, for every tumour size, mean VRHM/HT were respectively 1.49 ± 0.30, 1.05 ± 0.03 et 1.05 ± 0.10 for 177Lu, 1.04 ± 0.03, 1.01 ± 0.01 and 1.03 ± 0.17 for 90Y and 1.00 for 131I with standard deviations below 0.0044. Conclusion Results in our phantoms showed that L/ST interfaces have a minor impact on dosimetry of lungs and tumour bigger than 2 cm. Validation with clinical data containing intra-organ density heterogeneities will definitely validate the VSV dosimetry in lungs.
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