Primitive neuroectodermal tumor of the gallbladder.

Ewing sarcoma/primitive neuroectodermal tumor (ES/ PNET) most commonly occurs in the soft tissues of the extremities and the paravertebral areas. To our knowledge, no case of PNET has been reported in the gallbladder to date. We report a case of PNET of the gallbladder occurring in a 53-year-old woman. Microscopically, the monotonous small round cells with a formation of Homer-Wright rosettes showed intense membranous positive immunoreactivity for MIC2/CD99, as well as for neuron-specific enolase and synaptophysin. Other small round cell tumors, such as lymphoma, rhabdomyosarcoma, or melanoma, were excluded in light of negative immunoreactivity for leukocyte common antigen, desmin, and S100. The diagnosis of PNET was rendered based on the characteristic morphology and immunohistochemical findings, despite an absence of chromosomal translocations, such as t(11; 22)(q24;q12) and t(21;22)(q22;q12).