Malignant mesenchymoma, first described by Stout in 1948, is a malignant soft tissue tumor consisting of two or more distinct mesenchymal components, either or any of which might in itself be viewed as a primary malignant neoplasm (1
). Nowadays strict diagnostic criteria for malignant mesenchymoma require that each component is sufficiently differentiated histogenetically (6
). For this purpose, fibrosarcoma, hemangiopericytoma, malignant fibrous histiocytoma, myxosarcoma, and malignant peripheral nerve sheath tumor are not considered as separate malignant tumor components; those that normally fall into this category are liposarcoma, leiomyosarcoma, rhabdomyosarcoma, osteosarcoma, chondrosarcoma, and angiosarcoma (6
The previously reported radiologic features of malignant mesenchymoma vary (2
). Although none were common to all the cases they described, Suzuki et al. (5
) suggested that the typical findings of malignant mesenchymoma were large tumor size, a sharp margin, heterogeneous make-up, and massive calcification. The tumor in our case was also a huge heterogeneous mass with massive intratumorous calcification, but a feature different from those previously noted was also present. Earlier reports described a large mass with various intermingled components, whereas in our case the components were discrete. The massive calcification we found corresponded to an osteosarcomatous component, while the non-calcified enhancing nodules were, respectively, consistent with leiomyosarcoma, liposarcoma, and fibrosarcoma. A well-differentiated liposarcomatous component can have a fatty component (5
), though in our case none was apparent at CT. The composition of a malignant mesenchymoma - whether or not it contains calcification or ossification, a fatty component, or necrotic soft tissue - seems to determine its radiological features.
A malignant mesenchymoma is generally considered to be highly malignant, usually with a poor prognosis (7
), though the findings of this same study, based on clinicopatholgic analysis and long-term follow-up, suggest a more favorable prognosis. In our case, the clinical course was rather indolent; although the mass recurred locally 14 months after initial surgery, there was no evidence of metastasis during the 29 months following initial presentation, and this seems to reaffirm a more favorable prognosis.
For the proper diagnosis of malignant mesenchyoma, the radiologist must be aware of its nature; if any of the sarcomatous components are overlooked, it might be diagnosed as single-type or predominant sarcoma with divergent differentiation.
In summary, depending on its composition, malignant mesenchymoma may demonstrate various radiologic features. If a large retroperitoneal soft tissue mass is present, together with several different sarcomatous features such as prominent calcification/ossification or a fatty component, the differential diagnosis should include malignant mesenchymoma.