The scar sign: a useful finding on FDG PET/CT to distinguish sarcoidosis from other causes of lymphadenopathy.

Scar involvement is a rare but characteristic cutaneous manifestation of sarcoidosis. The concurrent presence of FDG-avid lymphadenopathy and scar involvement (the "scar sign") is a useful finding on FDG PET/CT to suggest sarcoidosis, especially when biopsy specimens are difficult to obtain. A 46-year-old woman who presented with fever, cough, and weight loss was found to have mediastinal and hilar lymphadenopathy on chest radiography and CT scan. FDG PET/CT scan showed FDG-avid lower cervical, mediastinal, hilar, and inguinal lymphadenopathy. There was also increased FDG uptake along an old hysterectomy scar. Mediastinoscopy and nodal biopsy revealed noncaseating granulomas compatible with sarcoidosis.