Morphologically Malignant Nodular Fasciitis With COL3A1::USP6 Fusion and aTP53 Mutation.

Abstract

Nodular fasciitis is a rapidly growing soft tissue tumor that typically arises in children and young adults. Histologically, it consists of short, sweeping, and intersecting fascicles of bland-appearing myofibroblasts. The majority of nodular fasciitis lesions harbor rearrangements involving the ubiquitin-specific protease 6 (USP6) gene. Most lesions are benign, with a tendency to regress spontaneously or remain stable following conservative excision. However, rare cases have shown clinically aggressive behavior, including distant metastasis. More recently, morphologic variants with malignant features have also been reported. We present a 20-month-old boy who developed a rapidly enlarging facial mass following blunt trauma. The excised mass revealed a variant of nodular fasciitis displaying marked cellular pleomorphism and frequent atypical mitotic figures. Molecular analysis identified a COL3A1::USP6 fusion and a TP53 Y234C mutation. While the majority of facial lesions referred to plastic surgeons for excision are benign, this case highlights the need for a broad differential diagnosis that includes both benign and potentially malignant entities. It also emphasizes the importance of multidisciplinary evaluation and management in patients with complex and unusual pathologic diagnoses.