FAPI-Based Theranostics in Medullary Thyroid Cancer: A Viable Alternative When Somatostatin Receptor-Based Theranostics Show Limited Efficacy-A Clinical Experience.

We present a case of a 36-year-old man with medullary thyroid carcinoma (MTC) who developed lung metastases 10 years post-total thyroidectomy. He presented with cough, and to evaluate his eligibility for peptide receptor radionuclide therapy ( PRRT ) , a 99m Tc-HYNIC-octreotide scan was conducted. The scan revealed multiple metastatic lymph nodes, lung metastases, and a solitary hepatic metastasis, all with mild octreotide avidity. Following 2 cycles of PRRT, minimal response was observed, and the lesions lost avidity. A subsequent 99m Tc-FAPI scan demonstrated favorable uptake in the metastatic lesions, including a non-octreotide-avid hepatic lesion, suggesting the potential utility of FAPI-based radiopharmaceuticals for MTC management.