Background: Anaplastic thyroid carcinoma (ATC) has historically been an almost uniformly rapidly fatal disease. In patients with loco-regional disease with no distant metastasis (stage IVB), and without a targetable mutation, a combination of external beam radiation delivered via intensity modulated techniques (IMRT) +/- concurrent cytotoxic chemotherapy is the current standard of care. However, the relapse rate is very high within the first year of diagnosis. Our data show that 90% of ATCs express PD-L1, with 43% having expression levels greater than 50%. A clinical trial with an anti-PD1 drug in ATC patients with active disease showed that patients with ATC had a response rate of 19%. Additionally, as tumor PD-L1 expression has been linked to radioresistance and there is a need to target micrometastatic disease, using adjuvant checkpoint inhibitor is a rational strategy in these patients. Methods: This is an open label, single center, phase 2 trial of adjuvant pembrolizumab after external beam radiation to the primary tumor in patients with stage IVB ATC. The primary cohorts will include those with gross, locoregional disease who have completed treatment with IMRT +/- concurrent radiosensitizing chemotherapy. Patients will be treated with adjuvant pembrolizumab (400mg IV Q6 weeks for up to 9 cycles), 2-6 weeks after completion of radiation. Patients will be enrolled based on dose of radiation in cohort 1 (high dose group, > 51 Gy), cohort 2 (palliative dose group, < 50 Gy). Cohort 3, consisting of those who underwent surgery followed by IMRT plus concurrent chemotherapy will be an exploratory cohort. Restaging scans will be performed every 12 weeks after enrollment. MRI brain will be repeated every 6 months in the absence of evidence of progression. The primary endpoint is median PFS (from the start of adjuvant pembrolizumab) in cohorts 1 and 2. Secondary endpoint is median overall survival in cohorts 1 and 2. Cohort 3 (5 patients) will have an exploratory endpoint, to estimate the median disease-free survival (DFS). Statistics: Survival analyses will be performed using the Kaplan-Meier method. Based on historical data, the median PFS for unresected patients is 6.8 months. For an expected improvement of median PFS by 3.7 months, with a combined total of 30 evaluable patients in cohorts 1 and 2, we anticipate having 80.0% power using 1-sided 10% alpha for a one-sample log rank test. The trial began enrollment in July 2022. This trial is sponsored by philanthropic funds and Merck. ClinicalTrials.gov ID NCT05059470. Clinical trial information: NCT05059470.