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. An excisional biopsy revealed a multilobular dermal tumor composed of areas of benign-appearing glomus tumor cells interspersed with areas of dense, pleomorphic cells arranged in sheets and fascicles with an elevated mitotic rate. The diagnosis of malignant glomus tumor was made on the basis of the lesion's nuclear atypia and elevated mitotic rate. This case demonstrates malignant transformation of a presumably benign glomus tumor precursor lesion highlighting this rarely reported phenomenon. The authors present this case to raise awareness of this vascular malignancy that carries a significant risk of metastasis.