The term ‘malignant mesenchymoma’ (MM) has traditionally been applied to a group of malignant soft tissue tumors that are characterized by the presence of two or more different mesenchymal tissue components in the same neoplasm. Currently, sarcomas with multilineage differentiation fulfilling the criteria of MM tend to be diagnosed as specific sarcomas with heterologous differentiation (e.g., de-differentiated liposarcoma) rather than MM; however, some cases of these tumors are difficult to classify. The purpose of this study was to analyze the clinicopathological and immunohistochemical features of these unclassifiable tumors, the so-called MM, and to determine whether some parameters are a useful adjunct for the classification of these tumors from the viewpoint of clinical outcome. Twelve cases of so-called MM were investigated retrospectively. The patients (six male, six female) ranged in age from 6 to 71 years (mean 48 years). Tumor size ranged from 3 to 20 cm (mean 13 cm). Tumor locations included the retroperitoneum (five cases), abdominal wall (two), thigh (two), forearm (one), lung (one), and parotid gland (one). The differentiated mesenchymal components were rhabdomyosarcomatous (RMS) (six cases), liposarcomatous (five), chondrosarcomatous (five), osteosarcomatous (five), or leiomyosarcomatous (four). Follow-up in all cases (range 4-129 months; mean 36 months) showed a 67% local recurrence rate, a 33% metastatic rate, and a 50% disease-related mortality. The patients who were under 40 years of age or who had an RMS component showed significantly worse survival than did patients in the other groups. The other mesenchymal components showed no significant correlation with survival. The immunophenotypes of p53 and mdm2 were almost identical between the two mesenchymal components in each case; however, there was no significant correlation between either of these immunophenotypes and the clinical outcome. None of the other factors (gender, tumor site, tumor size, or MIB-1-labeling index) was found to be a statistically significant prognostic indicator. In conclusion, a patient age under 40 years and the presence of an RMS component appear to be poor prognostic indicators of survival in patients with so-called MM, and to be a useful adjunct for the classification of these tumors.

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