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Infantile Hemangiopericytoma Versus Infantile Myofibromatosis Study of a Series Suggesting a Continuous Spectrum of Infantile Myofibroblastic Lesions

The clinicopathologic features of 11 tumors, originally diagnosed as infantile hemangiopericytomas and with a spectrum of morphologic findings, are described. The age of the patients ranged from 6 days to 7 years; seven patients were younger than 1 year (mean, 2.25 years; median, 10 months); six were boys and five were girls. Three neoplasms were situated in skin or subcutis and seven in deep soft tissue; in one case the depth was unstated. Seven lesions arose in the lower limbs, and one each in the lumbar region, clitoris, chest wall, and soft tissue of the zygomatic region. One patient later was found to have two additional dermal tumors, one each on the anterior abdominal wall and the chest wall. Follow-up information in eight patients revealed local recurrence 12 years later in one case only. Histologically, all tumors showed distinctive features of infantile hemangiopericytoma, including immature cytology, multilobulated growth pattern, focal necrosis, and mitotic activity in varying degrees. Vascular invasion was noted in seven cases. Additionally, a second tumor cell component, composed of spindleshaped myofibroblastic cells forming fascicles and micronodules, was evident at least focally. Both the spindle cells and more primitive round cells were positive for α-smooth muscle actin. Both cellular components showed a haphazard zoning arrangement. We discuss the clinicopathologic similarities between infantile hemangiopericytoma and infantile myofibromatosis and point out the differences between infantile and adult hemangiopericytoma. Our study suggests that there exists a broad spectrum of benign infantile myofibroblastic lesions containing an immature-appearing cellular component with a distinctive, hemangiopericytoma-like vascular pattern. Infantile myofibromatosis and so-called infantile hemangiopericytoma almost certainly represent different stages of maturation of the same (single) entity.