Pilomatrixoma, originally named “calcifying epithelioma of Malherbe,” was initially described in 1880 by Dr. Chenantals Malherbe as a calcified epithelioma of sebaceous glands.[1
] Developing in the subcutaneous tissue and arising from the matrix cells of the hair bulb, these benign tumors usually develop during the first two decades of life and have been very rarely described to degenerate to pilomatrixcarcinoma. The tumors are commonly found on the head with the cheek being a frequent location, followed by the neck and upper extremities in decreasing frequency. Pilomatrixomas are more seldom found on the trunk and lower extremities and are very rarely found in the breast.[2
] Despite a very low chance for malignant degeneration surgical excision is the treatment of choice as these tumors do not regress and occur in cosmetically unpleasant locations, usually the head and neck in a pediatric population. Surgery is curative and reoccurence after excision is rare.[1
To our knowledge, there have been only five reports of pilomatrixoma of the adult male breast in the English literature. The tumors were removed in two cases without undergoing medical imaging, most likely due to the size of the tumor.[3
] Another report of an unusually large pilomatrixoma described computed tomography and ultrasound findings.[5
] Two other reports described typical benign features of the tumor; one with mammography and sonography[6
] and the other with mammography alone.[1
] All prior case reports have emphasized that pilomatrixoma mimic breast cancer.
Sonography is routinely used to evaluate palpable masses as well as incidental masses found on screening mamography. Traditionally, ultrasound was used for differentiation of cystic from solid masses but its role has evolved to serve as a guide in characterization of masses into benign, indeterminate, or malignant categories. The negative predictive value for small breast masses with benign ultrasound features has been quoted to be greater than 99% in several studies.[7
] Features that have been described to most reliably characterize masses as benign are a round or oval shape, circumscribed margins, and width-to-anteroposterior dimension ratio greater than 1.4. According to Rahbar et al., if agreed upon by three different readers, these three features predicted benignity with 100% accuracy.[11
] The sonographic features in our case of male breast pilomatrixoma fit these criteria. In fact, pilomatrixomas on imaging are commonly well-defined subcutaneous nodules. Also, most are heterogeneously hyper- to isoechoic with acoustic shadowing and hyperechoic calcifications, as seen in our case.
In general, a differential diagnosis of a superficial breast lesion includes seborrheic keratosis, dermal nevus, epidermal inclusion cyst, and basal cell carcinoma. Much less frequently, dermal or subdermal lesions can represent breast carcinoma or hemangioma. Although the majority of superficial tumors are benign, management is frequently based on results of the triple test (physical exam, imaging, and biopsy) and biopsy is performed despite benign imaging characteristics.[8
] We suggest a conservative approach of interval follow-up for any small superficial breast mass with benign imaging characteristics. Such masses can be assigned into a BI-RADS 3 category without a pathologic diagnosis  or following a benign biopsy result to ensure stability over time. Also, conservative management can be considered in cases of pilomatrixoma of the adult breast. Although, most advise surgical excision of the tumor, especially in a pediatric population,[1
] an asymptomatic pilomatrixoma cosmetically acceptable to the patient seems safe to monitor with imaging as there is very low chance of malignant degeneration. Given the location and lack of symptoms, conservative management was discussed with our patient but excisional biopsy was ultimately performed due to anxiety resulting from a strong family history of breast cancer.
A well-circumscribed breast lesion on gray-scale ultrasound parallel to the skin surface has not been biopsied but is likely to be benign given its stability on sonography for nearly two years.