Intravascular papillary endothelial hyperplasia, also known as Masson tumor, is a benign reactive vascular proliferation that typically arises within a thrombus or pre-existing vascular lesion. Clinically, it may present as a red-blue papule or nodule and can mimic other benign or malignant vascular tumors. Microscopically, it is characterized by an intravascular proliferation of papillary, hyalinized fronds lined by bland endothelial cells, often admixed with fibrin and red blood cells. The process remains confined to the vascular lumen, lacks significant cytologic atypia, necrosis, and infiltrative growth, and should be distinguished from angiosarcoma.
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