SOLLID - a single centre study to develop methods to investigate the effects of low radiation doses within nuclear medicine, to enable multicentre epidemiological investigations

Alpha-Emitter Radiopharmaceuticals and External Beam Radiotherapy: A Radiobiological Model for the Combined Treatment

Previously published studies combined external beam radiotherapy (EBRT) treatments with different activities of ²²³Ra. The data of two-year overall survival (2y-OS) and neutropenia (TOX) incidence when combining EBRT and ²²³Ra are not homogeneous in literature. We adapted the linear-quadratic model (LQ) to ²²³Ra therapy using brachytherapy formalism for a mixture of radionuclides, considering the contribution of all daughter isotopes in the decay chain. A virtual cohort of patients undergoing ²²³Ra therapy was derived using data from the literature. The doses delivered using ²²³Ra and EBRT were converted into biologically equivalent doses. Fixed-effect logistic regression models were derived for both the 2y-OS and TOX and compared with available literature. Based on the literature search, four studies were identified to have reported the ²²³Ra injection activity levels varying from the placebo (0) to 80 kBq/kg, associated or not with EBRT. Logistic regression models revealed a dose-dependent increase in both the 2y-OS (intercept = -1.364; slope = 0.006; p-value ≤ 0.05) and TOX (-5.035; 0.018; ≤0.05) using the EBRT schedule of 8 Gy in 1 fr. Similar results were obtained for other schedules. Discrepancies between our TOX model and those derived for EBRT combined with chemotherapy are discussed. Radiobiological models allow us to estimate dose-dependent relationships, to predict the OS and TOX following combined ²²³Ra + EBRT treatment, which will guide future treatment optimization.