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.
Wang et al. (Fri,) studied this question.