MSH: A highly malignant subset of neoplasms arising from the endometrial stroma. Tumors in this group infiltrate the stroma with a wide range of atypia cells and numerous mitoses. They are capable of widespread metastases (NEOPLASM METASTASIS).,NCI: A malignant, mesenchymal tumor of the uterine stroma.,NCI: A malignant, infiltrating mesenchymal tumor arising from the uterine corpus, cervix, vagina, and the ovary. Based on its morphologic characteristics, it is classified as either a low grade or an undifferentiated (high grade) stromal sarcoma. The low grade endometrial stromal sarcoma is characterized by the presence of oval to spindle-shape cells that resemble the cells of the endometrial stroma, without evidence of significant atypia and pleomorphism. Numerous small vessels are also present. The undifferentiated stromal sarcoma is characterized by an aggressive clinical course, the presence of significant cellular atypia, pleomorphism, and high mitotic activity.