CD99-positive large cell neuroendocrine carcinoma with rearranged EWSR1 gene in an infant: a case of prognostically favorable tumor.

The first case of large cell neuroendocrine carcinoma arising in an infant is presented. The tumor arose at the anal verge of a 1-year-old girl. The diagnosis of this CD99-positive tumor was supported by expression of epithelial (keratins, EMA, and Ep-CAM) and neuroendocrine (chromogranin A, synaptophysin, and neuron-specific enolase) markers and absence of immunoreactivity for Fli-1. No fusion of EWSR1 with FLI-1 or ERG was detected by polymerase chain reaction. However, the split of the EWSR1 gene was demonstrated by fluorescence in situ hybridization. This case adds to the few epithelial tumors in which an EWSR1 rearrangement was demonstrated. Because the tumor was initially misclassified as an extraskeletal Ewing's sarcoma, the patient was treated according to the Ewing's sarcoma treatment protocol. She remains free of tumor 8 years after initial diagnosis.