Monophasic cellular variant of infantile myofibromatosis. An unusual histopathologic pattern in two siblings.

Infantile myofibromatosis (IMF) is a distinct clinicopathologic entity characterized by solitary or multicentric tumors present at birth or in early infancy with a typical biphasic (central hemangiopericytoma-like, peripheral leiomyoma-like) histologic pattern. We present a case of IMF in two siblings with onset of disease in late childhood. Histology of the primary as well as several later tumors was characterized by a monophasic cellular appearance with a prominence of tiny capillaries initially suggesting an unusual vascular tumor. Diagnosis was established by the development of more characteristic biphasic lesions during the course of disease. Immunocytochemistry (Ulex, factor VIII, JC/70A [CD31], PAL-E, BMA120, EN4, QBEnd10 [CD34], SMS actin) and ultrastructural studies showed no (marked) differences between different types of IMF. The monophasic cellular pattern should be recognized as an unusual histologic manifestation of IMF, in particular in patients outside the classical setting or presentation.