The Combination of Immunotherapy and Stereotactic Ablative Radiotherapy in MSS Oligometastatic Colorectal Cancer

Immune-checkpoint inhibitor (ICI) has led to a paradigm shift in the treatment of patients with metastatic cancer, as proved by improved survival and durable responses in a group of these patients. However, the response rates to ICI when given alone are limited. In gastrointestinal cancer, patients with microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) tumors shown a response rate of approximately 40% to ICI, while patients with microsatellite stable (MSS) or mismatch repair-proficient (pMMR) tumors respond poorly to ICI. Such observations have spurred efforts to expand the benefit of immunotherapy, especially in these immune "non-sensitive" tumors, by combining ICI with treatments that induce T-cell associated immune response such stereotactic ablative radiotherapy (SABR). High-dose ablative radiation was showed to facilitate immunotherapy through promote the activation of innate and adaptive immune responses against tumors in preclinical model and early phase clinical trials. However, a large portion of trials which were launched to test the efficacy of radiotherapy and immunotherapy produced suboptimal results. One important reason could be that majority of these trails were designed with single lesion irradiation, which is insufficient to unveil enough tumor antigens and/or to break the barrier of immunosuppressive tumor microenvironment (TME). The investigators hypothesized that irradiation to multiple or all sites of diseases is more likely to produce an optimized regimen with ICI by broadly stimulate anti-tumor immunity and reduce tumor burden. Therefore, this study plans to administrate SABR to as many metastatic lesions as possible, in combination with ICI (Sintilimab) in patients with MSS oligometastatic colorectal cancers, to assess safety and tolerability of the regimen, and evaluate its early efficacy as well.