Intramuscular myxoma. A clinicopathologic, immunohistochemical, and electron microscopic study.

This clinicopathologic study concerns 17 cases of intramuscular myxoma, including an immunohistochemical survey of 10 cases and an electron microscopic examination of 4. There was a female preponderance in a ratio of 14:3. The most common sites of tumors were the large muscles of the thigh (seven cases), followed by those of the buttock (three) and the lower leg (three). The size of the tumor ranged from 1.5 to 20 cm (median, 6 cm) in the greatest diameter. Neither recurrence nor metastasis was seen in any of 15 patients for whom information was available. In addition to the conventional microscopic features, such as hypocellularity, absence of a plexiform capillary network, and no detection of typical glycogen-rich lipoblasts, the following findings were regarded as helpful to differentiate an intramuscular myxoma from myxoid liposarcoma: hypovascularity of the tumor, demonstrated by angiography; a homogeneous computed tomography appearance with low density, absence of S-100 protein immunoreactive cells such as lipoblasts; and electron microscopically, the constituent cells were predominantly fibroblast-like cells with a prominent secretory activity, together with a small number of primitive mesenchymal cells and histiocyte-like cells, but with no lipoblasts. After simple excision, the 15 patients who could be followed are well with no recurrence during various periods of follow-up.