Study of Cemiplimab Combined With Dabrafenib and Trametinib in People With Anaplastic Thyroid Cancer

Inclusion Criteria: * Pathological (histologically or cytologically) proven diagnosis of BRAF-V600E mutant ATC (a diagnosis that is noted to be consistent with ATC is acceptable) * Either Metastatic disease or locoregional disease that is considered not resectable for cure * Ideally a surgeon should determine that the disease is not resectable for cure, but this can also be done by any investigator * Patients must have measurable disease according to RECIST 1.1 criteria, defined as at least 1 lesion that can be accurately measured in at least 1 dimension (longest diameter to be recorded for nonnodal lesions and short axis for nodal lesions) as \>/= 20 mm with conventional techniques or as \>/= 10 mm with spiral CT scan, MRI, or calipers by clinical exam * Age \>/= 18 years * Eastern Cooperative Oncology Group (ECOG) performance status \</= (or Karnofsky performance score \>/= 60) * Able to swallow and retain orally administered medication * Patient must have normal organ and marrow function as defined below: * Absolute neutrophil count \>/=1.5 x 10\^9/L * Hemoglobin \>/=8 g/dL * Platelets \>/=100 x 10\^9/L * Serum bilirubin \</=1.5x institutional ULN (unless the patient has GIlbert's Disease, in which case total bilirubin \</=3x institutional ULN) * AST and ALT \</=2.5x institutional ULN (\</=5x institutional ULN if there is liver metastasis) * Serum creatinine \</=1.5mg/dL or calculated creatinined clearance (Cockcroft-Gault formula) \>/=50 mL/min or 24-h urine…