Malignant proliferating trichilemmal tumor in the skin over the breast: a case report.

A proliferating trichilemmal tumor is relatively uncommon. It is composed of multiple cysts consisting of squamous epithelium with trichilemmal keratinization without granular layer interposition. This lesion usually occurs in the scalp of elderly women. We describe a 67-year-old woman with a malignant proliferating trichilemmal tumor in the skin over the breast. We first misdiagnosed the disease as a primary squamous cell carcinoma of the breast with a metastatic lymph node in the axilla because of the disease site and our unfamiliarity with the disease. The patient underwent radical mastectomy with axillary dissection. Eight months postoperatively, a tumor appeared in her right axilla and progressively enlarged. We subsequently excised the tumor. She is healthy as of 8 months postoperatively. To the best of our knowledge, only one case of a proliferating trichilemmal tumor occurring in the skin over the breast has been reported.