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

SIMPLE SUMMARY: The linear–quadratic (LQ) model was adapted to (223)Ra therapy using brachytherapy formalism for a mixture of radionuclides, considering the contribution of all daughter isotopes in the decay chain. The LQ model allowed us to predict the two-year overall survival and neutropenia rates with a combination of external beam radiotherapy and (223)Ra treatment. The fitted model could be used to guide future optimization and personalization of combined treatments. ABSTRACT: Previously published studies combined external beam radiotherapy (EBRT) treatments with different activities of (223)Ra. The data of two-year overall survival (2y-OS) and neutropenia (TOX) incidence when combining EBRT and (223)Ra are not homogeneous in literature. We adapted the linear–quadratic model (LQ) to (223)Ra 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 (223)Ra therapy was derived using data from the literature. The doses delivered using (223)Ra 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 (223)Ra 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 (223)Ra + EBRT treatment, which will guide future treatment optimization.