A Rare Case of Adenocarcinoma With Micropapillary Structure and Apocrine Differentiation Arising in a Sebaceous Tumor.

A 77-year-old Japanese man presented to our hospital with subcutaneous tumors of the right upper arm and axilla. A biopsy revealed a cutaneous adnexal tumor, showing apocrine differentiation, and axillary lymph node metastasis. After chemoradiotherapy to shrink the tumors, both lesions were resected. A resected specimen of the arm tumor showed a variegated histology: (1) a classic sebaceoma with an organoid pattern and sebocytes; (2) a sebaceous tumor with cellular atypia; (3) a papillotubular tumor showing a biphasic pattern of pale eosinophilic cells with apocrine differentiation and basaloid cells; and (4) an invasive adenocarcinoma with a micropapillary structure, reminiscent of an invasive micropapillary carcinoma of the breast. The axillary tumor was regressed. To our knowledge, this is the first reported case of an adnexal tumor of the skin with an invasive micropapillary structure arising in a sebaceous tumor.