Thyroiditis Induced by Immune Checkpoint Inhibitors Exhibits High [68Ga]Ga-FAPI-46 and [18F]FDG Uptake on PET/CT

A 69-year-old man diagnosed with sarcomatoid pleural mesothelioma received 3 cycles of nivolumab with 2 cycles of ipilimumab. Treatment was terminated after the patient developed colitis-an immune-related adverse event (irAE). High-dose prednisolone was initiated. Between 1 and 2 weeks later, the patient underwent FDG- and FAPI PET/CT, both exhibiting prominent, diffuse uptake in the thyroid gland. Biochemistry just before the scans revealed hypothyroidism with further derangement in the following weeks. The patient was diagnosed with irAE thyroiditis. FAPI uptake in chronic thyroiditis has been previously described, whereas uptake in irAE thyroiditis has only been reported once.