Infiltrative glomus tumor arising from a benign glomus tumor: a distinctive immunohistochemical pattern in the infiltrative component.

Malignant glomus tumors (MGT) are rare. Although metastatic MGT has been reported, most MGT have only been locally aggressive, some with multiple local recurrences. We report an additional case of an infiltrative glomus tumor. In addition to the pattern of immunohistochemical staining for alpha-smooth muscle actin (SM-actin) previously described, we performed immunohistochemical stains for Ki-67 and CD34. The infiltrative component of the glomus tumor showed variably decreased staining with SM-actin and occasional tumor cells showed nuclear staining with Ki-67. CD34 staining occurred in stromal cells forming the pseudocapsule in the benign component of this tumor and in other benign glomus tumors. The infiltrative component showed increased CD34 stromal cells. Although Ki-67 staining showed only an occasional proliferative cell, the immunohistochemical staining pattern of CD34 and SM-actin raise the possibility that the infiltrative component of this tumor may have differences in the degree of differentiation from the circumscribed part and that local factors could support its spread from a conventional benign glomus tumor.