Primitive myxoid mesenchymal tumor of infancy presenting with an abdominal and thoracic mass misdiagnosed as neuroblastoma: A case report.

Primitive myxoid mesenchymal tumor of infancy (PMMTI) is a rare soft tissue tumor that has been recently described in only a few cases. A 10-month-old boy was admitted with a mass filling the abdomen and right hemothorax. Gross examination revealed a fragile, and occasionally gelatinous and rubbery mass. On microscopic examination, there were fibrillary off-white appearance with occasionally myxoid microcystic areas. Immunohistochemical examination results were as follows: BCOR (+), and SATB2 (+) with diffuse nuclear expression, Desmin (-), CD34 (-), S-100 (-), MyoD1 (-), Myogenin (-). The next-generation sequencing showed internal tandem duplication of the BCOR exon 15. In conclusion, PMMTI is a distinctive entity with unique characteristics. However, it can be clinically confused with other childhood malignancies such as neuroblastoma. Recurrence can be seen despite chemotherapy in patients with microscopic residue.