Unusual Case of Concurrent Retroperitoneal Congenital Infantile Fibrosarcoma and Cellular Type Congenital Mesoblastic Nephroma

Background: Although congenital infantile fibrosarcoma (cIFS) is a rare soft tissue sarcoma among children, it constitutes one of the most common soft tissue sarcomas during the first year of life. Congenital mesoblastic nephroma (CMN) is the most common benign renal tumor usually developing during the first 3 months of life. cIFS and cellular type CMN (cCMN) share not only similar histopathologic features but identical molecular genetic abnormality including the ETV6/NTRK3 fusion gene. Here, we report an unusual case of cIFS occurring with cCMN. Case Presentation: An 18-month-old girl presented with a 1-month history of abdominal distension and a few days' history of a palpable abdominal mass. A large heterogenous mass sized 9.0x11.2x11.6 cm on the right side of the abdomen and an isolated heterogenous lesion sized 4x4.5 cm within the right kidney were noted from the imaging study. Pathologic findings were consistent with cIFS and cCMN of the right kidney. In addition, both pathologic specimens contained the ETV6/NTRK3 fusion gene. Conclusion: Although cIFS and cCMN share similar histopathologic features and molecular genetic abnormality, simultaneous occurrence of these 2 types of tumor is exceedingly rare. To our knowledge, this is the first unusual case report of concurrent cIFS and cCMN.