Retroperitoneal infantile fibrosarcoma: clinical, molecular, and therapeutic aspects of an unusual tumor.

The authors describe a patient with a large retroperitoneal infantile fibrosarcoma that responded well to preoperative chemotherapy, which subsequently facilitated the complete surgical resection of the mass. The patient had an unusual site of metastasis presumed to be to a regional lymph node. The histology on initial core biopsies was not classic but showed a round cell, myxoid pattern. The presence of the t(12;15)(p13;q25) and the fusion transcript ETV6-NTRK3 by RTPCR facilitated the diagnosis.