Phase II Trial of Pembrolizumab in Metastatic or Locally Advanced Anaplastic/Undifferentiated Thyroid Cancer

Inclusion Criteria: * Histologically- or cytologically-confirmed diagnosis of anaplastic thyroid cancer (ATC) or undifferentiated thyroid cancer (UTC). A diagnosis of possible ATC/UTC will be allowed if the clinical presentation is consistent with anaplastic or undifferentiated thyroid cancer. * Disease characteristics one of the following: * Unresectable ATC/UTC limited to the neck: Subjects must have received radiation therapy or surgery to primary tumor and have subsequent evidence of ATC/UTC. * Metastatic ATC/UTC: either with entirely surgically-removed cancer/metastatic only disease, or with disease in the neck not requiring radiation or surgery to the neck mass * Measurable disease per RECIST v1.1. Lesions situated in a previously-irradiated area are considered measurable if progression has been demonstrated in such lesions. * ≥ 18 years of age. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 (within 7 days prior to the first dose of pembrolizumab). * Absolute neutrophil count (ANC) ≥ 1500/µL (within 10 days prior to the first dose of pembrolizumab). * Platelets ≥ 100 000/µL (within 10 days prior to the first dose of pembrolizumab). * Hemoglobin ≥ 9.0 g/dL or ≥ 5.6 mmol/L (within 10 days prior to the first dose of pembrolizumab). Criteria must be met without erythropoietin dependency and without packed red blood cell (pRBC) transfusion within last 2 weeks. * Creatinine ≤ 1.5 × ULN OR Measured or calculated creatinine clearan…