TPS817 Background: Metastatic disease remains a consistent challenge for patients with gastrointestinal (GI) malignancies. Multiple immune checkpoint inhibitors (ICI) have been FDA approved for several GI indications. Still, patients will progress on these therapies, either immediately or after initial response. Palliative radiation therapy (RT) is an effective treatment that can be delivered to symptomatic metastases. It also has been reported to lead to an abscopal effect, which is an induction of response in a distant tumor. This is thought to result from increased tumor immunogenicity, tumor microenvironment modulation, and immune cell recruitment. Thus, palliative RT could not only relieve symptoms, but also potentially reinvigorate the immune response, prolong ICI efficacy, and delay next-line systemic therapy that carries risks of unknown efficacy and tolerability. Ongoing studies seek to elucidate prognostic and predictive biomarkers of the abscopal effect. However, cohorts of patients with GI malignancies are limited, so there remains a need for research in combining RT and immunotherapy in the context of metastatic GI cancer. ARM-GI aims to study the radiation-augmented immune response in patients with metastatic GI cancers progressing on immunotherapy. Methods: ARM-GI is a phase 2, single arm study. Key eligibility criteria include confirmed metastatic GI malignancy, progressive disease per RECIST v1.1 on any ICI, and at least 2 metastases, one of which can be safely left unirradiated. The target prescription dose is 30 Gy in 5 fractions to symptomatic sites through intensity modulated RT or stereotactic body RT. Patients will continue the same immunotherapy after RT and be evaluated per RECIST v1.1 longitudinally. The primary endpoint is overall response rate (ORR) per RECIST v1.1. Secondary endpoints include ORR per iRECIST, progression free survival, overall survival, local control in radiated lesion(s), effect of distant radiation on unirradiated target lesions, incidence of new metastatic lesions, safety by CTCAE v5.0, and time to new systemic therapy. The study will enroll 28 patients, with recruitment ongoing. Serial peripheral blood samples are collected for exploratory studies that will analyze for association with disease response and quantification of changes in circulating immune cell subsets after RT compared to baseline. Clinical trial information: NCT04221893 .