Malignant mesenchymoma. Clinicopathologic analysis of a series with evidence of low-grade behaviour.

Malignant mesenchymoma is a rare soft tissue neoplasm which, by definition, shows at least two distinct types of frankly malignant mesenchymal differentiation in addition to any "undifferentiated" or "fibrosarcomatous" areas. It is generally regarded as a high-grade tumor. A series of nine new cases is presented in which all patients except one were adults. Four tumors arose in the retroperitoneum, three in the thigh. The only definable constituents were rhabdomyosarcoma (eight cases), liposarcoma (seven cases), and osteo/chondrosarcoma (five cases). On histologic grounds, at least one element in each case would normally be regarded as high grade. Of six cases with a median follow-up of 4.2 years, only one has died from tumor. One further case was an incidental postmortem finding. In contrast to most current opinion, these data suggest that malignant mesenchymoma is not as aggressive as the histology would imply.