Congenital/infantile fibrosarcoma of the colon: morphologic, immunohistochemical, molecular, and ultrastructural features of a relatively rare tumor in an extraordinary localization.

Fibrosarcomas diagnosed during the early years of life are called congenital/infantile fibrosarcomas. They differ from adult fibrosarcomas because of their limited aggressive outcome. Congenital/infantile fibrosarcomas occur most frequently on the extremities. This article describes an exceptional case of colonic congenital/infantile fibrosarcoma diagnosed in a 3-day-old baby boy. It is the third intestinal congenital/infantile fibrosarcoma reported in the international literature. The lesion was radically excised. Microscopic examination revealed a densely cellular and poorly circumscribed tumor composed of spindle cells forming interlacing fascicles with herringbone appearance. Necrotic and hemorrhagic areas were appreciable. Mitotic count was 2/10 high-power fields. Immunohistochemistry revealed that the tumor cells were positive for vimentin, focally positive for h-caldesmon, and that they were negative for epithelial markers, muscular markers, S-100 protein, and CD34. The proliferation index (Mib-1) was 15%. Polymerase chain reaction demonstrated the chromosomal translocation t(12;15) (p13;q25). At the ultrastructural level, neoplastic cells had fibroblastic and myofibroblastic features. The patient underwent follow-up without adjuvant therapy. Twelve months after the surgery, he is alive and well. Given the common indolent nature of this tumor, it is important to avoid misdiagnoses with more aggressive tumors. The algorithm for the diagnosis of congenital/infantile fibrosarcoma, especially outside the usual localizations, should comprise morphologic, immunohistochemical, molecular, and ultrastructural studies.