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Fig. 1.46 Nerve sheath tumour of maxillary nerve. The nerve (arrow) at the entrance to foramen rotundum is enlarged compared to opposite side. This is similar to the appearance expected for perineural spread of malignancy. Note the carotid artery (arrowhead).

head and neck sarcomas {1231,1562}, arising de novo or less commonly In the setting of neurofibromatosis type 1 (NF1) {1059,1231,1795}. De novo MPNST peaks in the fourth decade, while those in the setting of NF1 occur at an earlier age. There is a female predominance for de novo sinonasal MPNST {1041}, and a male predominance in NF1-associated MPNST {774}.

Etiology

Radiation and possibly immunosuppression may be etiologic factors {1562}

Localization

They commonly arise from the ophthalmic and maxillary branches of the trigeminal (5th) cranial nerve, but can involve all of the sinonasal tract and nasopharynx {756,1153,1795,2018}.

Clinical features

Presenting symptoms include mass, pain, epistaxis, deviation or swelling of tonsils, nasal obstruction, and sinusitis {27,1319,1891,1964,2119}.

Macroscopy

MPNST is generally globoid to fusiform, pseudoencapsulated, cream-grey and firm, occasionally associated with surface ulceration. Infiltration into the surrounding soft tissues and bone is common. The tumours are often large (>5 cm) and may be attached to a nerve. Foci of cyst formation, necrosis and/or haemorrhage are frequent.

Tumour spread and staging

Local extension into contiguous structures along the path of the trigeminal nerve or through the foramen ovale are characteristic {1452}. MPNSTs metastasize to the lungs, bones, and/or liver, {1041} while the epithelioid variant tends to involve regional lymph nodes {1437}.

Histopathology

MPNSTs can either be spindled (95%) or epithelioid (5%) {1437}. At low magnification, both types show alternating areas of dense cellularity with less cellular myxoid areas. Geographic necrosis and perivascular accentuation of tumour cells are common. The tumour cells are fusiform and plump, arranged in tightly packed fascicles woven into a vague "herringbone" pattern, while in other areas the cells are wavy with fibrillar cytoplasmic extensions, arranged in a loose myxoid

Necrosis Nose
Fig. 1.47 Epithelioid malignant peripheral nerve sheath tumour (MPNST) of nasal cavity. Tumour composed of short fascicles of plump spindly or polygonal cells.

Fig. 1.48 Epithelioid malignant peripheral nerve sheath tumour of nasal cavity.

Fig. 1.49 Malignant peripheral nerve sheath tumour. A Increased cellularity with focal haemorrhage and hyperchromatic and atypical nuclei. B Highly cellular tumour with atypical spindle cell population with slightly 'wavy' nuclei. Note focal necrosis at the lower right corner.

background matrix. Focal palisading of nuclei may be present. The tumour cells are variably pleomorphic, with a high nuclear to cytoplasmic ratio and mitotic activity. Many sinonasal tract MPNSTs, in contrast to those occurring in other anatomic sites, are histologically and biologically low-grade {1041}. An origin from a nerve may or may not be apparent. MPNST with rhabdomyoblasts are known as malignant Triton tumours.

Immunohistochemistry

The spindle cell variant is usually focally positive for S100 protein and occasionally positive for glial fibrillary acidic protein (GFAP). However, up to 30% of MPNST may be negative for S100 protein {2784A}. The epithelioid variant is diffusely immunoreactive for S100 protein and may mimic malignant melanoma, {583,756,2603} but other melanoma markers are negative. In malignant Triton tumour, the rhabdomyoblasts are posi tive for desmin and other skeletal muscle markers.

Differential diagnosis

The differential diagnoses include synovial sarcoma, fibrosarcoma, spindle cell carcinoma, leiomyosarcoma and mucos-al malignant melanoma {1041,2550, 2603}.

Precursor lesions

MPNST may arise from neurofibroma (especially in the setting of NF1) and only exceptionally from classic schwannoma.

Somatic genetics

Both NF1 alleles must be inactivated for MPNST to occur in NF1. Malignant progression from neurofibroma in NF1 is related to alterations of genes controlling cell cycle regulation, including TP53 {1459} and CDKN2A (which encodes p16) {1361,1895}.

Genetic susceptibility

MPNST of the sinonasal tract may be associated with NF1, typified by germline mutation of the NF1 tumour suppressor gene located on chromosome 17 {454}.

Prognosis and predictive factors

Surgery is the treatment of choice, although radiation and chemotherapy may have a palliative role. De novo sinonasal MPNSTs have a 5-year survival rate of about 90%, which is superior to that of 50-65% for MPNSTs arising in other anatomic locations

{1041,1562,2715}. However, NF1-associ-ated sinonasal MPNSTs have a 5-year survival rate of only about 15% {2119}. Poor prognostic factors include male gender, high tumour cellularity and high mitotic activity {1041}.

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