IMRT Followed by Pembrolizumab in the Adjuvant Setting in Anaplastic Cancer of the Thyroid (IMPAACT): Phase II Trial Adjuvant Pembrolizumab After IMRT in ATC

Primary Objective: -To estimate the median progression-free survival (PFS) (from the start of adjuvant pembrolizumab until locoregional progression, development of distant metastatic disease, or death) in stage IVB ATC patients with gross disease, treated with external beam radiation (+/- concomitant chemotherapy) followed by adjuvant pembrolizumab. Patients will be patients enrolled from cohort 1 and 2 (cohort 1: ≥51 Gy; cohort 2: ≤50 Gy). Secondary Objective: -To estimate median overall survival in stage IVB ATC patients treated with external beam radiation (+/- concurrent chemotherapy) followed by adjuvant pembrolizumab. Patients will be stratified by dose of external beam radiation. Exploratory Objective: -To estimate the median disease-free survival (DFS) in patients with stage IVB ATC treated with surgery/external beam radiation/concomitant chemotherapy followed by adjuvant pembrolizumab (cohort 3).