Primary cutaneous histiocytoid carcinoma with distant metastasis.

Distinguishing primary cutaneous adnexal carcinoma from metastatic carcinoma of unknown primary can be a diagnostic challenge due to the frequent overlap of histologic and immunohistochemical features. A 58-year-old man presented with a tender, indurated plaque on axillary skin. Biopsy revealed infiltrating atypical cells throughout the dermis, without connection to the epidermis. Tumor cells had a histiocytoid appearance and displayed mild pleomorphism. The tumor was discohesive and had areas with a single file pattern. Signet ring cells were also present. Cells were reactive with CK7, CK5/6, p63, GATA3, GCDFP-15 and Her 2-neu. Additional studies were negative, including TTF-1, CDX2, E-cadherin, mammaglobin, estrogen receptor and progesterone receptor. Thorough clinical and radiologic evaluation failed to identify an occult primary extracutaneous malignancy; however, regional lymphadenopathy, widespread osteoblastic lesions and multiple subcentimeter liver hypodensities were noted. Considering the clinical and histopathologic features, the diagnosis of primary cutaneous histiocytoid carcinoma with distant metastasis was favored.