Dual-Tracer PET/CT in Erysipelas-like Cutaneous Metastases From Salivary Duct Adenocarcinoma: FAPI Illuminates What FDG Missed

Cutaneous metastases are rare manifestations of malignancies. We report a case of a 50-year-old man with an atypical presentation of erysipelas-like skin lesions, later found to be cutaneous metastases from an unknown primary. 18F-FDG PET/CT revealed visceral metastases; however, it failed to demonstrate uptake in the skin lesion, while 68Ga-FAPI PET/CT additionally showed intense uptake in the skin, including the right parotid gland, which is likely the primary site. IHC findings confirmed a primary salivary duct adenocarcinoma. This case underscores FAPI PET/CT's utility in detecting FDG-negative lesions and identifying occult primaries in atypical presentations, also reflecting the intratumoral heterogeneity of salivary duct carcinomas.