Intestinal infantile fibrosarcoma with translocation of NTRK. A case report and review of the literature.

Introduction: Infantile fibrosarcoma is a rare non-rhabdomyosarcomatous soft tissue tumor (0.0005%) of which only 10% occur in the abdomen where they rarely affect the gastrointestinal tract. The median age at diagnosis is 3 months although 40% of them are present at birth.

Material And Methods: When infantile fibrosarcoma is diagnosed in our center, a clinical-pathological description is made together with a bibliographic review.

Results: We present the case of a 6-day-old girl who presented with irritability and rejection of food. She was diagnosed with acute abdomen due to perforation and underwent surgery where a mass on the ascending colon was removed. Histopathology revealed a proliferation of spindle cells consisting of intertwined fascicles, infiltrating the adjacent tissues. Nuclear pleomorphism, few mitoses, foci of necrosis and hemorrhage are seen. Immunohistochemistry showed positivity for Pan-TRK and the NGS panel (Archer DX) demonstrated the TPR::NTRK1 fusion. No case with these characteristics, location or TPR::NTRK1 fusion were found in the literature.

Conclusions: Infantile fibrosarcoma is a very infrequent tumor which is exceptionally rare in the intestine. It is important to look for the characteristic genetic rearrangement of these tumors both to confirm the diagnosis and differentiate them from other pediatric spindle cell tumors and determine the correct targeted treatment. Selective TRK inhibitors have shown a 75% response rate in children and adults with tumors that exhibit TRK fusion. It was possible to find fusions with the Archer DX panel that the Oncomine panel did not detect.