Only a few cases have been reported in adults. Typical gross findings are similar to other soft tissue tumors and sarcomas. The microscopic features vary from those of a small cell, undifferentiated tumor to those with readily identifiable neural or mesenchymal elements. As noted previously, the most common mesenchymal element is rhabdomyosarcoma. Neural elements run the gamut from neuroblastoma to ganglioneuroma.
Tumors can be locally aggressive or have nodal and hematogenous metastases.
The aggressiveness may correlate with the predominant phenotype, but this hypothesis has not been tested. The most important prognostic factor appears to be resectability. MEM is an interesting neoplasm, in that the potential for maturation of its two usual components, myogenous and neural, mirrors the potential maturation of rhabdomyosarcoma and neuroblastoma into rhabdomyoma and ganglioneuroma, respectively. This modulation, usually occurs post chemotherapy, although spontaneous maturation has been reported. In our case, the primitive spindled elements seen in the original biopsy appear to have modulated into a tumor having both rhabdomyoblasts, and ganglion-like cells.