Purpose or Case Report: Pilomatricomas (PM), benign neoplasms of the hair follicle matrix cells, present in the pediatric population with a slight female predominance. These neoplasms are the second most commonly excised pediatric skin mass and comprise up to 10% of superficial masses evaluated by pathologists. Typically, they present as a slow growing, solitary, painless, superficial mass in the head and neck.

We present a unique case of trauma preceding rapid development of an abnormally large PM. No cases with such characteristics currently exist in the literature.

A preschool aged girl presented with a two-month history of an enlarging superficial scalp mass following local trauma. The parents reported that she was running, fell, and hit her head on the ground. The patient reported minor pain, discoloration and local swelling, but was otherwise well with no loss of consciousness. One week post-injury, the family noticed a persistent “mosquito-bite sized bump” at the site of scalp trauma. Over the next two months, the “bump” steadily enlarged to the size of a ping pong ball, prompting evaluation. The patient denied associated pain, itching, or drainage. Exam noted a well-appearing girl with a round, well-marginated, firm, painless 3.0 cm mass at the left frontotemporal scalp.

Given the reported trauma, CT was performed and demonstrated a superficial scalp mass measuring 2.8 x 3.1 cm with internal calcifications and no underlying fracture or bony involvement. Sonographic evaluation showed a well-marginated, heterogeneous mass with a hypoechoic rim and internal doppler flow revealing spectral arterial and venous waveforms. MRI revealed a peripherally enhancing mass with foci of internal susceptibility. Core needle biopsy yielded fragments of epithelium composed of follicular matrix cells and aggregates of shadow cells.

A successful surgical excision was performed yielding the final pathologic diagnosis of a PM. She had an uncomplicated postoperative course.

Our case illustrates a unique case of PM, in which there was an atypical history of preceding trauma and rapid enlargement, resulting in clinical uncertainty. Unfortunately, PM’s are frequently misdiagnosed or under-considered by radiologists on initial ultrasound evaluation, despite being a common entity. Our case underscores the importance of maintaining a high suspicion for PM when evaluating a superficial mass in pediatric patients. This may reduce unnecessary and invasive procedures in the pediatric patient population.
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