Objective: To highlight the morphological, immunohistochemical, and molecular features of a pediatric schwannoma.

Methods: Archival material of a pediatric patient diagnosed with schwannoma was retrieved and reviewed, focusing on the clinical, histological, and molecular features.

Data and results: The patient is an 11-year-old with a history of neuroblastoma, diagnosed at nine months of age, with no MYCN amplification or 1p loss. They presented with a soft tissue lesion on the left middle finger, dorsal aspect. Ultrasound imaging demonstrated a 2.8 x 0.8 x 0.8 cm lesion overlying the dorsal aspect of the base of the digit near the metacarpophalangeal joint. Histology revealed well-circumscribed nodules surrounded by relatively thin EMA-positive perineural capsules, arranged in a plexiform/multinodular-type growth pattern. Each nodule consists of lesional cells arranged in short fascicles/bundles with occasional clefting and internal nodule formation with a distinct 'serpentine' palisading pattern. The lesion consists of Antoni A regions with Verocay body formation. There are no significant Antoni B areas. The spindle/ovoid cells (Schwannian cells) are bland with elongated and tapered nuclei, showing strong and diffuse positivity for S100. There is no significant cytologic atypia, mitotic figures, degenerative/cystic changes, or necrosis. There are large nerve bundles associated with the lesion in the surrounding soft tissue. TruSight RNA Pan- Cancer sequencing identified SH3PXD2A::HTRA1 gene fusion.

Conclusion: We characterized a pediatric schwannoma harboring a SH3PXD2A::HTRA1 gene fusion with the distinctive serpentine histology. According to the literature, the expression of this fusion results in elevated phosphorylated ERK, increased proliferation, invasion, and in vivo tumorigenesis.

Aida Glembocki received her MD from the University of Buenos Aires and completed her pathology residency at Hospital General de Agudos Dr. Ignacio Pirovano. As a pediatric pathologist, she was part of the molecular pathology section of the pathology department at Hospital Dr. Juan P. Garrahan (Argentina). She was a pathology fellow at The Hospital for Sick Children (SickKids) and is completing a Master of Science in Clinical Epidemiology & Health Care Research at the Institute of Health Policy, Management and Evaluation, University of Toronto.