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Pathogenesis of hemangioma.

Hemangioma is the most common tumor of any kind seen in infancy. It is also, perhaps, the least understood. This stems in part from a long history of confusing nomenclature for vascular anomalies that employed classification schemes based on superficial descriptions of the lesions. The first major step toward clarifying this nomenclatural morass was made by Mulliken and colleagues (1, 2), who employed a biological classification scheme based on the differing clinical courses and endothelial proliferative activity of hemangiomas versus malformations. Hemangiomas, often called infantile or juvenile hemangiomas for clarity, are benign tumors that exhibit an early and rapid proliferation phase during the first year of life characterized by endothelial and pericytic hyperplasia, followed by a slower but steady involution phase that may last for years. This basic scheme of histopathological classification has been refined by immunohistochemical characterization of hemangioma in each of the phases (3). Nonetheless, the pathogenesis of hemangioma is still not understood. Although growth factors and hormonal and mechanical influences have been postulated to affect the abnormal proliferation of endothelial cells in hemangioma, the primary, causative defect in hemangiogenesis remains unknown and no genetic alteration has been implicated.