Answer: Infantile fibrosarcoma

Histology: A spindle cell tumor diffusely involves the affected bowel wall. This spindle cell tumor is highly cellular, shows mitotic activity and features non-pleomorphic cells with ovoid to plump nuclei and indistinct cell borders. Fascicles are poorly formed. Dilated blood vessels are additionally evident. The tumor was non-immunoreactive for desmin, myogenin, CD34, C-kit (CD117), and ALK.

Discussion: Inflammatory myofibroblastic tumors (IMT) of childhood may overlap somewhat histologically with infantile fibrosarcoma. However, IMT are generally characterized by a more prominent inflammatory infiltrate and less densely packed myofibroblastic tumor cells. Some IMT are associated with chromosome abnormalities (specifically, translocations) involving chromosome 2p23, the location of the ALK gene. These translocations result in a disruption of ALK expression, which can be detected using immunohistochemistry for ALK protein. Embryonal rhabdomyosarcoma would have more eosinophilic cytoplasm, may demonstrate cross striations, and would be immunoreactive for desmin and myogenin. Gastrointestinal stromal tumors are typically tumors of adults, which would stain for CD34 and, more consistently, C-kit.
Infantile fibrosarcoma (also referred to as congenital fibrosarcoma) is distinctive among soft tissue sarcomas in that it morphologically looks like a high-grade sarcoma, yet clinically behaves like a low-grade sarcoma. Only approximately 30% of cases recur and only 5% metastasize. In contrast, morphologically similar adult type fibrosarcomas metastasize in 50% of cases. The distinctive clinical behavior of infantile fibrosarcoma was recently corroborated by demonstration of a distinctive genetic abnormality that is specific to this tumor. This is a t (12;15) (p13;q25) translocation that results in the fusion of the ETV6 gene to the NTRK3 gene, resulting in a chimeric fusion transcript. The resulting fusion protein is thought to result in constitutive activation of NTRK3, a neurotrophin receptor that has tyrosine kinase activity. The identical translocation has been identified in the cellular variant of congenital mesoblastic nephroma of the pediatric kidney, suggesting that this latter entity represents infantile fibrosarcoma affecting the renal sinus.