SS18::POU5F1 Fusion–Associated Sarcoma of the Shoulder Presenting with Lymph Node Metastases in a 5-Year-Old Male: A Case Report

Fusion-associated sarcomas are rare, high-grade soft tissue malignancies. The SS18::POU5F1 fusion has been described only in a small number of pediatric cases. A healthy 5-year-old boy presented with a progressively enlarging, erythematous left shoulder mass initially thought to represent a hematoma or abscess. Ultrasound suggested a soft tissue abscess; however, surgical exploration revealed a solid mass. Gross excision yielded a 4.5 × 4 × 3 cm lesion weighing 22 g. Histopathology demonstrated an SS18::POU5F1 fusion–associated poorly differentiated malignant neoplasm, with two lymph nodes positive for metastasis. Staging MRI confirmed axillary nodal involvement. The patient subsequently underwent axillary lymph node dissection, re-excision of the primary site due to margin concerns, and tunneled catheter port placement, after which he was treated with risk-adapted adjuvant chemoradiotherapy per the ARST0332 Children’s Oncology Group protocol with planned longitudinal surveillance imaging and multidisciplinary follow up. Fusion-associated sarcomas must be included in the differential diagnosis of soft tissue masses in children.