Welcome back to Rad Onc Smart Review, Head and Neck edition.This is Episode 4: Unknown Primary — The Neck Mass Without a Mucosal Home.In the last episode, we talked about the workup. We scoped, we scanned, and we biopsied. Most of the time, that process finds the culprit — a small tonsil cancer hiding in crypts or a base of tongue tumor you could miss on casual inspection. But sometimes, you do everything right — contrast imaging, PET, examination under anesthesia, directed biopsies, palatine tonsillectomy, and even tongue base sampling — and the pathology still comes back negative for a primary site. Now you have metastatic squamous cell carcinoma in a cervical lymph node and no mucosal home address. That is Unknown Primary.