A Monte Carlo approach to small-scale dosimetry of solid tumour microvasculature for nuclear medicine therapies with 223Ra-, 131I-, 177Lu- and 111In-labelled radiopharmaceuticals

Abstract

The small-scale dosimetry of radionuclides in solid-tumours is directly related to the intra-tumoral distribution of the administered radiopharmaceutical, which is affected by its egress from the vasculature and dispersion within the tumour. The aim of the present study was to evaluate the combined dosimetric effects of radiopharmaceutical distribution and range of the emitted radiation in a model of tumour microvasculature.We developed a computational model of solid-tumour microenvironment around a blood capillary vessel, and we simulated the transport of radiation emitted by 223Ra, 111In, 131I and 177Lu using the GEANT4 Monte Carlo. For each nuclide, several models of radiopharmaceutical dispersion throughout the capillary vessel were considered.Radial dose profiles around the capillary vessel, the Initial Radioactivity (IR) necessary to deposit 100 Gy of dose at the edge of the viable tumour-cell region, the Endothelial Cell Mean Dose (ECMD) and the Tumour Edge Mean Dose (TEMD), i.e. the mean dose imparted at the 250-μm layer of tissue, were computed. The results for beta and Auger emitters demonstrate that the photon dose is about three to four orders of magnitude lower than that deposited by electrons. For 223Ra, the beta emissions of its progeny deliver a dose about three orders of magnitude lower than that delivered by the alpha emissions.Such results may help to characterize the dose inhomogeneities in solid tumour therapies with radiopharmaceuticals, taking into account the interplay between drug distribution from vasculature and range of ionizing radiations.