A Periadrenal Gland Ewing Sarcoma With Extensive Posttreatment Ganglioneuroblastoma-Like Morphology in an Adult Patient.

Neuroblastoma and Ewing sarcoma are two round cell malignancies that can exhibit light microscopic overlap and potentially arise in the same anatomic location, leading to diagnostic uncertainty. Neoadjuvant chemotherapy-induced cytodifferentiation/maturation is a common feature in pediatric malignancies, whereas Ewing sarcomas often exhibit variable degrees of necrosis, inflammatory infiltrates, and stromal fibrosis after neoadjuvant treatment. Posttreatment neuroblastoma-like morphologic changes have been rarely reported in pediatric Ewing sarcoma. Herein, we report a 33-year-old man with a retroperitoneal periadrenal mass and normal plasma catecholamine levels. The tumor biopsy demonstrated Homer-Wright rosette-forming small round cells with a high nuclear-to-cytoplasmic ratio and finely granular chromatin. The tumor cells showed strong, diffuse membranous CD99 and nuclear NKX2.2 expression, whereas they were negative for PHOX2B, a neuroblastoma/ganglion cell marker. EWSR1 rearrangement detected by FISH confirmed the light microscopic impression of Ewing sarcoma. The postchemotherapy resection showed a biphasic tumor with a predominantly ganlioneuroblastoma-like morphology and a minor component of round cell sarcoma. The round cell sarcoma area retained membranous CD99 but was negative for NKX2.2. The ganglioneuroblastoma-like areas retained NKX2.2 immunoreactivity, remained negative for PHOX2B, and were additionally negative for CD99. EWSR1 rearrangement by FISH was observed in the resection specimen. Our report shows a rare example of chemotherapy-induced cytodifferentiation resembling ganglioneuroblastoma in an adult patient with conventional Ewing sarcoma.