Squamoid Eccrine Ductal Carcinoma of the Thumb

Summary: A 78-year-old woman presented to orthopedic surgery with a 4cm intermittently tender, firm nodule of the left thumb that had grown rapidly three years prior before stabilizing. Excisional biopsy revealed nests and cords of eosinophilic squamoid cells in a hyalinized dermis with foci of ductal differentiation and lumen formation, with an overlying in situ intraepidermal component containing atypical keratinocytes. Perineural invasion of a 0.05mm-diameter nerve was present. Tumor was present at all margins. Immunohistochemistry revealed expression of CK5/6 and p53 in lesional cells, suggesting a primary cutaneous tumor rather than a metastatic adenocarcinoma. Ductal cells were positive for CEA. CK7, S100, ER, and D2-40 were negative. Based on histopathology and immunophenotype, a diagnosis of squamoid eccrine ductal carcinoma (SEDC) was made. SEDC is a rare, locally aggressive tumor with morphologic and molecular features intermediate between squamous cell carcinoma and sweat gland carcinomas. Lymph node metastases have been reported in ~14% of described cases, highlighting the importance of timely diagnosis. There is no consensus regarding optimal management of SEDC. Wide local excision, amputation, Mohs surgery, and sentinel lymph node biopsy are currently being considered for our patient, with final treatment plans pending multidisciplinary consultation.