Malignant Triton Tumor
This specimen is from a patient with neurofibromatosis 1. The patient gave history of rapid growth in a long-standing thigh mass. The tumor showed areas with conventional histologic features of a malignant peripheral nerve sheath tumor (MPNST), poorly-differentiated areas resembling high-grade sarcoma, as well areas with clear-cut rhabdomyosarcomatous differentiation (seen in this touch prep from freshly cut tumor surface; note abundant rhabdomyoblasts with deeply eosinophilic cytoplasm). The spindle cells were strongly S-100 positive, whereas the rhabdomyoblasts were positive for desmin and myogenin. Divergent differentiation (osteochondromatous, skeletal, angiosarcomatous, glandular, neuroendocrine) is seen in about 15% of MPNST. Most are associated with NF1. MPNSTs with rhabdomyosarcomatous areas are termed Malignant Triton Tumor. They are highly aggressive tumors with a 5-yr survival rate of 10-15% in contrast to about 50% 5-yr survival for conventional MPNSTs. Rhabdomyosarcoma can also be seen as a component of other tumors, including dedifferentiated liposarcoma, medulloblastoma, germ cell tumors, melanoma, ectomesenchymoma and in GIST following therapy.