Intramyofiber skeletal muscle invasion in Ewing's sarcoma of bone: clinicopathologic observations from the intergroup...

Skeletal muscle may be infiltrated by a variety of invasive or metastatic malignant tumors. In 14 out of 291 cases of Ewing's sarcoma, we observed diagnostic biopsies in which local skeletal muscle fibers contained intrasarcolemmal aggregates of tumor cells. Cases demonstrating local intramyofiber invasion have a higher mitotic count, an increased rate of developing distant metastases, and a decreased survival when compared to cases not demonstrating this skeletal muscle "parasitism." The usefulness of skeletal muscle intramyofiber invasion in the differential diagnosis of Ewing's sarcoma remains unknown at this time.