A Retrospective Study of the Clinicopathological Characteristics of Approximately 1,600 Pilomatricomas Treated at a Single Institution.

Background: First reported by Malherbe in 1880, pilomatricoma is a common benign skin tumor generally believed to occur mainly in children and adolescents. We conducted this study to better define the characteristics of pilomatricoma and compare our findings with current knowledge.

Methods: Patients diagnosed pathologically with pilomatricoma from 2016 through 2020 at Nippon Medical School Musashi Kosugi Hospital were included (1,559 patients, 1,590 tumors). Clinicopathological characteristics were analyzed.

Results: The male to female ratio was 1:1.6, and the most common tumor site was the upper limbs (33.7%). Preoperative diagnosis was correct in 48.5% of the patients, and their average age at resection was 33.5 years. Resection was carried out in 70% of the patients within 1 year, but time to resection was more than 1 year in the other 30%. Pathologically, squamous stratifying keratinocytes were observed in 41.7% of the patients, cells with a large pale pink cytoplasm in 38.9%, hair papilla-like structures in 33.9%, ossification in 15.7%, trichohyalin granules in 11.9%, and aggregations of follicular germinative cells in 7.8%. Of the chronological and morphological stages proposed by Kaddu (stage 1: early, stage 2: fully developed, stage 3: early regressive, stage 4: late regressive), stage 3 was the most common (70.6%).

Conclusion: Pilomatricoma is more common in females, regardless of ethnicity or age, but the tumor location in the upper limbs commonly observed in Japanese patients may indicate ethnic differences. Pathologically, the fact that cells linked to follicular differentiation are observed suggests that pilomatricoma is a complex panfollicular neoplasm. Time to resection appears to correlate with Kaddu stages. Factors such as age, location, sex, depth, and stage may affect the pathological features of this tumor.