Malignant glomus tumor in the branchial muscle of a 16-year-old girl.

Malignant glomus tumor is an extremely rare neoplasm and its histological features are not well characterized. We report a 16-year-old female patient with a malignant glomus tumor. The patient was admitted to our hospital presenting with a mass in the right upper arm that she had noticed for the previous 6 months. Computed tomography and magnetic resonance imaging revealed an expanded mass involving the surrounding tissues. At surgery, an ill-defined and expanded mass was found, 5 x 4 x 3 cm in size, in the right branchial muscle. The tumor was extirpated, along with neighboring muscle tissues. Histologically, tumor cells were round to short-spindle shaped, forming solid sheets admixed with vessels of varying size. Their nuclei were uniformly oval to round, and their cytoplasms were slightly eosinophilic. The growth pattern of the tumor cells resembled that of glomus tumor, but mitotic figures were frequent (as high as 10 per 10 high-power fields). Immunohistochemically, the tumor cells were positive for vimentin and muscle actin, but negative for desmin. There were no areas typical of benign glomus tumor or sarcomatous change. These findings led us to a diagnosis of primary malignant glomus tumor arising de novo. There has been no recurrence or metastasis for 21 months after wide excision.