Malignant Glomus Tumor Arising in a Long Standing Precursor Lesion.

Malignant glomus tumor is an exceedingly rare vascular malignancy of unclear etiology, which is believed to arise de novo or from malignant transformation of a benign glomus tumor. The authors present a case of an elderly woman evaluated for a painful 2.5-cm purple-brown tumor on the right forearm that grew rapidly within a long-standing, seemingly benign lesion.