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Washington University Experience | NEOPLASMS - CRANIAL AND PARASPINAL NERVEs | Malignant Peripheral Nerve Sheath Tumor (MPNST) | 12G MPNST, triton tumor (Case 12) A3 Ki67 1.jpg
12G The overall Ki-67 proliferation index is very high (~70%). ---- Not shown: A subset of neoplastic cells also show nuclear positivity for p53, albeit with variable intensity (moderate to strong), and focal positivity for CD57 (cytoplasmic). A neurofilament stain supports its solid growth pattern in cellular areas. Comment The collective findings are those of a 'high-grade' malignant peripheral nerve sheath tumor (MPNST) with multifocal rhabdomyosarcomatous differentiation; presence of latter qualifies it for a diagnosis of malignant triton tumor (MTT). Divergent (mesenchymal) differentiation, as seen in this case, is in fact frequently associated with neurofibromatosis. Further, presence of necrosis in this tumor prompts a WHO grade IV designation. In general, MTT is more aggressive than MPNST and has a worse prognosis. Other worrisome findings seen in this particular case are its large size (>5 cm), positive resection margins, high Ki-67 proliferation index (>25%), and p53 immunoreactivity.
