Radiobiology: Foundation and New Insights in Modeling Brachytherapy Effects.

Fundamentals in radiobiology commonly known as the '4R's' concept (ie, reoxygenation, repair, redistribution, and repopulation) have mostly been investigated for external beam radiotherapy. However, these fundamentals can be applied to brachytherapy (BT) by accounting for differences in dose rate, fractionation, and response to immunologic agents for this treatment modality. Many improvements have been achieved in the era of dosimetric optimization but still limited data are available regarding radiobiological opportunities for BT. As BT is characterized by a large degree of dose heterogeneity, a wide range of dose rates and fractionations exist within the implanted volume. Calculations based on the linear quadratic model can be used to estimate the dose-response equivalence between various BT modalities. Such models are helpful in daily practice and open possibilities in terms of radiobiological optimization. However, some limitations should be highlighted in terms of the applicability of the linear quadratic model. Furthermore, in vitro models do not account for the complex interplay between the tumor and its microenvironment, including vascularization and/or immune response. Recently, an improved understanding of the tumor's microenvironment has led to investigations of immunomodulatory agents in combination with radiotherapy. BT is a promising candidate to enhance the immunogenic response with concomitant immunotherapy. This review summarizes some of the main mechanisms involved in tissue response to BT. Preclinical data, clinical evidence, as well as novel approaches to radiobiology are highlighted.