FAPI Uptake in Avascular Necrosis of the Talus.

We present a case of metastatic castration-resistant prostate cancer (mCRPC) with prior hepatic involvement who ultimately received 177 Lu-PSMA therapy. During the posttreatment scan following the third cycle, atypical uptake was observed in the left ankle, which was also evident as FAPI uptake on the 68 Ga-FAPI-46 PET/CT performed for restaging, and was subsequently confirmed as avascular necrosis through MRI correlation. This case illustrates the false-positive uptake of 177 Lu-PSMA-617 and 68 Ga-FAPI-46 in avascular necrosis of the talus.