Ossifying pilomatricoma and a novel hypothesis for its pathogenesis: A case report and comprehensive literature review

RATIONALE: Pilomatricoma is a benign skin appendageal tumor derived from hair follicle matrix cells that commonly affects the head, neck, and upper extremities of the pediatric population. Since the original tumor description, diverse variants have been reported in the literature. Pilomatricoma with florid osseous metaplasia is described as an ossifying pilomatricoma and is recognized as a distinct variant of this benign tumor. However, the pathogenesis of this variant remains unclear. In this study, we present an uncommon case of ossifying pilomatricoma and address the pathogenesis of metaplastic ossification through a comprehensive literature review. PATIENT CONCERNS: A 14-year-old boy presented with an asymptomatic protuberant mass in the preauricular region. DIAGNOSIS: Based on its clinicopathological features, we diagnosed the lesion as an ossifying pilomatricoma. INTERVENTIONS AND OUTCOMES: The lesion was surgically removed under local anesthesia. The postoperative course was uneventful during the 6-month postoperative follow-up. LESSONS: We suggest that metaplastic ossification in ossifying pilomatricoma represents another feature of foreign body reaction to keratinous materials containing shadow cells in old lesions and a walling-off phenomenon to prevent exposure of surrounding tissues to keratinous materials.