Interdigitating Dendritic Cell Sarcoma in Upper Arm Misdiagnosed as Ruptured Epidermal Inclusion Cyst

Interdigitating dendritic cell sarcoma (IDCS) is an extremely rare neoplasm that usually arises in lymph nodes, but also can exist in extranodal sites. This report is about a case study of IDCS in the upper arm skin with axillary lymph node metastasis. A 66-year old woman had a slowly growing mass with tenderness sensation on her right upper arm that was being injected triamcinolone acetonide at local clinic. The presumptive diagnosis was a ruptured epidermal inclusion cyst, and empiric antibiotic therapy was done; however, she had poor respondence to antibiotic therapy. Therefore, magnetic resonance imaging, incisional biopsy, positron emission tomography-computed tomography, and bone scan were performed and a malignant tumor was diagnosed. She received surgical resection and lymph node dissection of the right axilla. No adjuvant chemotherapy was done. Although extremely rare, this case suggests that extranodal IDCS should be considered in differential diagnosis of untreated atypical skin mass and early biopsy should be performed.