Phosphaturic Mesenchymal Tumor in the Foot Detected by 99m Tc-FAPI-46 SPECT/CT and 18 F-FDG PET/CT

A 58-year-old woman presented with disabling bone pain and muscle weakness. Laboratory tests were unremarkable, except for decreased serum phosphate and elevated urine phosphate levels. A 99m Tc-MDP scan revealed multiple sites of bone fractures. Both 18 F-FDG PET/CT and 99m Tc-FAPI-46 scans identified a lesion in the left heel, which was later resected and diagnosed as phosphaturic mesenchymal tumor on pathologic evaluation. This case represents a pioneering experience in utilizing FAPI for detecting the origin of tumor-induced osteomalacia. It underscores the potential utility of various radiopharmaceuticals in the diagnostic workup of these patients.