Malignant peripheral nerve sheath tumour (MPNST) of the sinonasal tract is a very rare neoplasm  that is probably under-recognised due to the lack of reproducible histologic criteria and to the tendency of these tumours to be negative for the commonly used immuno-histochemical markers of nerve sheath differentiation. Only in some cases can the diagnosis be based on the identification of a pre-existing neurofibroma. Histologically, MPNST is a moderately to highly cellular spindle cell proliferation, with variable mitotic activity and areas of necrosis. A variant composed of epithelioid cells has been described in the sinonasal cavities (Fig. 2.18) . Some tumours may show morphologic and immuno-histochemical features of skeletal muscle differentiation and are designated "malignant triton tumours" .
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