Malignant glomus tumor of the urinary bladder.

We present a case of a malignant glomus tumor arising in the urinary bladder of a 57-year-old woman with metastatic pulmonary nodules who died 2 months later. Pathologically and clinically confirmed malignant glomus tumors are exceedingly rare, especially those that arise in the visceral organs. The present case retained its architectural similarity to a benign glomus tumor and consisted of sheets of highly malignant round cells showing cytoplasmic positivity for smooth muscle actin. On reticulin histochemical staining, we found that reticulum fibrils surrounded individual tumor cells, suggesting cellular investment by basement membrane. We discuss the concept of malignant glomus tumors and emphasize the criteria that distinguish them from other malignant tumors.