Unexpectedly Low FDG and FAPI Uptake in Mucosal Melanoma: Case Report with Literature Review.

Mucosal melanomas (MMs) represent a rare subset of melanomas arising from melanocytes in noncutaneous sites such as mucosal membranes of the head-and-neck, gastrointestinal, and genitourinary tracts. Unlike cutaneous melanomas, MMs often present late due to their occult nature, leading to a poor prognosis. Fluorine-18 fludeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) is widely utilized for staging and monitoring melanomas, leveraging their characteristic high FDG avidity. Recently, fibroblast activation protein inhibitor (FAPI)-based PET/CT imaging has become a popular modality in the evaluation of various cancers. Here, we present a case of MM where FDG avidity was notably absent and mild FAPI avidity, despite the lesion's typical anatomical location and clinical suspicion. This case highlights an uncommon scenario within the literature and underscores the variability in radiotracer uptake observed in MMs, challenging conventional imaging expectations and potentially impacting clinical management decisions.