Ectopic hidradenoma papilliferum of the breast: ultrasound finding.

Hidradenoma papilliferum (HP) is a benign neoplasm arising from mammary-like glands which typically involves the dermal layer of the female anogenital area. The prognosis for HP is good. Recurrence is unusual and is typically attributed to incomplete excision of the primary tumor. Malignant transformation is rare and HP of the breast has not yet been reported. Ectopic HP is usually solitary, small, and asymptomatic. It appears as a well-circumscribed, complex cystic mass in the dermis on ultrasound. We present a case of HP arising from the axillary tail of the breast.