18F-FDG PET/CT Monitoring of Response to Programmed Death Protein 1 Blockade in Metastatic Basal Cell Carcinoma.

A 73-year-old woman with history of metastatic basal cell carcinoma presented with a large spinal mass demonstrating a high mutational burden of 111 mutations/Mb. She underwent T12 corpectomy and T10-L3 posterior spinal fusion followed by adjuvant radiation. After 2 years of surveillance, FDG PET/CT and a contrast-enhanced MRI revealed recurrence, prompting salvage therapy with the hedgehog signaling pathway inhibitor, sonidegib, which only transiently stabilized the disease. An off-label treatment with the programmed death protein 1 inhibitor, pembrolizumab, was initiated. Serial FDG PET/CT showed gradual decline in tumor metabolism over the next 12 months, leading to complete metabolic response.