Morphologic Overlap between Infantile Myofibromatosis and Infantile Fibrosarcoma: A Pitfall in Diagnosis.

Infantile myofibromatosis (IM) is a distinctive mesenchymal disorder with different clinical forms, including solitary, multicentric, and generalized with visceral involvement. A wide morphologic spectrum is encountered, with the extremes resembling congenital infantile fibrosarcoma (CIFS) and infantile hemangiopericytoma. We report a series of lesions with mixed features of CIFS and IM and compare them in order to further define their clinicopathologic features and the significance of the so-called composite fibromatosis. Seven lesions with unusual overlapping morphologic "composite" features of both IM and CIFS were selected from a series of 106 myofibroblastic lesions. Three cases classified as composite infantile myofibromatoses (COIM) were highly cellular tumors with a diffuse growth of primitive mesenchymal cells and focal features of IM combined with areas resembling infantile fibrosarcoma (IF). Four cases were classified as IF. Three of these exhibited a biphasic pattern with foci resembling IM, including whorls of primitive and spindle cells and perivascular and intravascular projections of myofibroblastic nodules, and the 4th had a close histologic resemblance to a primitive, immature IM. With reverse transcriptase polymerase chain reaction, the ETV6-NTRK3 transcript was absent in 3 COIM and was detected in 3 CIFS; the other CIFS had typical cytogenetic aberrations. On the basis of currently available information, COIM represents a morphologic variant of IM that can mimic IF. Careful histologic evaluation to detect the typical features of IM is essential to avoid classification as IF. Molecular analysis for the ETV6-NTRK3 gene fusion is an important diagnostic tool in this group of lesions.