Malignant cutaneous glomus tumor presenting as a rapidly growing leg mass in a pregnant woman

A 21-year-old pregnant woman presented with a rapidly growing >2 cm nodule on her right leg, involving dermis and subcutaneous tissue. Histologically, the tumor was composed of sheets and nests of neoplastic cells with variable cytomorphology, including typical round to ovoid glomus cells with clear cytoplasm and well-defined cell borders, small cells and spindle cells. Numerous medium to large vessels were present throughout the tumor. Moderate- to high cellularity, nuclear atypia and frequent mitotic figures (42 MF/50 High power field (HPF)) were noted. Immunohistochemistry showed cytoplasmic and membranous expression of actin (HHF-35) and membranous expression of type IV collagen. The histologic features and immunoprofile were consistent with the diagnosis of malignant glomus tumor, a rare soft tissue neoplasm that typically arises on the extremities. Histologic features that infer malignancy in glomus tumors include the combination of large size (>2 cm) and deep location, or atypical mitotic figures, or moderate to severe cytologic atypia with high mitotic activity (>5 mitoses /50 HPF). Although our case was superficially located, the nuclear atypia and mitotic rate, as well as the large size, fulfilled the criteria for a malignant glomus tumor.