Impact of radiotherapy dose, fractionation and immunotherapeutic partner in a mouse model of HR+ mammary carcinogenesis

Background Hormone receptor (HR)+ breast cancer is poorly responsive to immune checkpoint inhibitors (ICIs). In some settings, radiation therapy (RT) has been shown to mediate immunostimulatory effects and promote ICI sensitivity. Methods We investigated whether hypofractionated RT may be successfully combined with ICIs in a mouse model of multifocal, metachronous HR+ mammary carcinogenesis. We hypothesized that focal RT targeting the first detectable (primary) tumor combined with ICIs may generate effective immunity, hence delaying the development of new lesions. Results Focal RT in various doses and fractionations limited primary tumor growth, with an optimum for a 20 Gy X 2 regimen (ablative in ∼90% of mice). The degree of primary disease control, however, did not necessarily correlate with overall survival (OS) extension, owing to changes in the development of new neoplastic lesions contributing to global tumor burden. Adding a PD-1 blocker to focal RT delivered in a 10 Gy X 3, 20 Gy X 2 or 8 Gy X 6 regimen failed to alter OS extension enabled by RT alone. Similar results were obtained with a CTLA4 blocker, an IL1B inhibitor, and a PD-1 blocker plus recombinant FLT3LG when combined with the 10 Gy X 3 regimen. Conclusions In this model of HR+ mammary carcinogenesis, RT to the primary tumor ameliorates OS (to an extent depending on dose and fractionation). Increasing local control by RT alone or RT plus immunotherapy beyond a hitherto undefined threshold, however, does not necessarily inhibit the development of subsequent non-irradiated neoplasms and hence does not necessarily provide extra OS benefits.