Low-Grade Malignant Proliferating Tricholemmal Tumor Associated With Occult Ductal Carcinoma In Situ of the Breast.

Malignant proliferating tricholemmal tumor (MPTT) is a rare adnexal neoplasm most commonly located on the scalp. We a 62-year-old woman who presented to her local physician with a palpable but not visible nodule of the left breast, initially diagnosed as squamous cell carcinoma. Histopathological evaluation of the reexcision specimen incidentally revealed ductal carcinoma in situ. On referral to our institution, the skin biopsy specimen was reviewed and interpreted as a low-grade MPTT. This patient and 2 previously reported cases of MPTT involving breast skin highlight presentation of a rare tumor at an unusual site and the potential for misdiagnosis as squamous cell carcinoma. Reexcision provided the unexpected benefit for our patient of early diagnosis of ductal carcinoma in situ not revealed by imaging.