Cutaneous Metaplastic Synovial Cyst: Case Report and Literature Review from the Dermatological Point of View.

Cutaneous metaplastic synovial cysts (CMSCs) are rare tumors typically comprising a solitary, well-circumscribed cystic mass that is not connected to the joint. Synovial cysts have been reported predominantly by orthopedists or pathologists; however, the presence of CMSC is not generally well recognized by dermatologists. Herein, we report a CMSC in a 68-year-old woman receiving systemic corticosteroid therapy for the treatment of eosinophilic granulomatosis with polyangiitis (EGPA). We attempt to delineate the clinical characteristics of this unusual neoplasm by reviewing the literature, focusing especially on dermatological descriptions. Histologic examination of the surgical specimen in the current case revealed that the cystic wall was lined with layers of flattened synovial cell-like cells and connective tissues, mimicking the synovial membrane. Positive immunoreactivity of the lining cells against vimentin was detected, but no immunoreactivity against cytokeratin, carcinoembryonic antigen (CEA), CD68, or S-100 was detected. The pathogenesis of CMSC remains unclear, but it has been tightly linked to direct traumatic stimuli or relative tissue fragility, which potentially accounts for CMSC development in our case. Most CMSCs reported by dermatologists are located on the extremities, whereas those described by other specialists tend to be distributed more globally. Preoperative diagnoses are often either epidermal cyst or suture/foreign body granuloma. Incomplete surgical excision of usual synovial cysts may lead to local recurrence, which has been reported in oral and maxillofacial surgery, but not in dermatologic surgery. This fact could be explained by the technical difficulties of surgical excision related to anatomical location. Dermatologists need to be aware of CMSC, and CMSC should be included in the differential diagnosis of subcutaneous cysts.