Widespread Skeletal Metastases in Medullary Thyroid Carcinoma

A 78-year-old woman with medullary thyroid carcinoma (MTC) demonstrated persistently elevated serum calcitonin despite prior surgery and radiotherapy. 99mTc-octreotide imaging failed to detect skeletal metastases, while 68Ga-FAPI PET/CT revealed extensive skeletal involvement. Biochemical markers were disproportionately low relative to disease burden, consistent with a low-secretory MTC. This case highlights the limitations of SSTR-targeted imaging in MTC and underscores the diagnostic superiority of FAP-targeted imaging.