Generalized infantile myofibromatosis with a monophasic primitive pattern.

Infantile myofibromatosis (IM) is a rare disorder present at birth or in early infancy with a biphasic histological pattern. We present a neonatal-onset case of generalized IM with visceral (central nervous system, heart, lungs, liver, spleen, small intestine, kidneys and bones) and placental involvement, showing a monophasic histological pattern through the lesions during the course of disease. Histologically, the tumor was composed of a solid proliferation of cytologically uniform, 'primitive' mesenchymal cells associated with a hemangiopericytoma-like vascular pattern. Immunohistochemical analysis and ultrastructural study revealed that the tumor cells exhibited primitive features without mature myofibroblastic differentiation. Neither ETV6-NTRK3 nor ACTB-GLI fusion gene was identified. The patient died of cerebral hemorrhage and respiratory failure at four months of age despite intensive therapy. Generalized IM characterized by monophasic primitive pattern could be related to poor clinical outcome.