Pathology of Malignant Peripheral Nerve Sheath Tumour

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Dr Sampurna Roy MD 2022

The term 'Malignant peripheral nerve sheath tumour' (MPNST) is used by most authors (previously called neurosarcoma, neurogenic sarcoma, neurofibrosarcoma and malignant schwannoma).

This name indicates the possibility that the tumour may have diverse origins, some arising from Schwann cells, others from perineurial cells and still others from fibroblasts of the nerve sheath.

Individuals with internal plexiform neurofibromas are 20 times more likely to have MPNSTs than individuals without internal plexiform neurofibromas.

Age: Usually occurs in young and middle aged adults.

Site: Commonly occurs in deep soft tissues of the proximal extremities.

Clinical presentation:

i) Sporadic cases ii) 50% cases associated with Neurofibromatosis iii) 10% cases radiation induced

Gross features:

These are large tumours, more than 5 cms in diameter.

The tumour arises as large fusifom mass within a major nerve.

It has a fleshy, white tan surface.

Focal areas of hemorrhage and necrosis may be present.

Most cases are deeply situated, however some cases may develop in superficial neurofibroma.

Histopathological features:

The tumour is composed of spindle cells arranged in sweeping fascicles.

Dense cellular areas alternate with hypocellular, myxoid zones.

Geographic areas of necrosis with tumour cells palisading at the edges.

The spindle cells in some areas show nodular or whorled appearance.

There is perivascular accentuation and proliferation of tumour cells within the subepithelium of blood vessels.

Grade of the tumour depends on the cellularity and mitosis

Foci of osteogenic sarcoma, chondrosarcoma, angiosarcoma or rhabdomyosarcoma is present in about 15% cases (Malignant Triton tumour).

Rarely there may be foci of glandular differentiation.

Variants of Malignant Peripheral Nerve Sheath Tumour:

-Myxoid Malignant peripheral nerve sheath tumour :

-Epithelioid Malignant Peripheral Nerve Sheath Tumour:

" Epithelioid malignant peripheral nerve sheath tumor (EMPNST) is rare and differs from conventional malignant peripheral nerve sheath tumor by showing diffuse S-100 protein positivity, infrequent association with NF1, and occasional origin in a schwannoma. Loss of INI1 expression is seen in a subset of tumors." Epithelioid malignant peripheral nerve sheath tumor: clinicopathologic analysis of 63 cases.

This variant consists of epithelioid cells with vesicular nuclei and prominent nucleoli.

Focal spindle areas are also present.

Immunohistochemistry: A strong cytoplasmic and nuclear staining with S 100 protein. Focal staining with Smooth Muscle Actin. Other stains are usually negative.

Differential diagnosis: Amelanotic melanoma (HMB45 is negative in epithelioid MPNST)

-Pigmented (Melanotic) Malignant Peripheral Nerve Sheath Tumour:

The tumour displays epithelioid cells with vesicular nuclei and grooved nuclei, abundant melanin pigment and numerous mitotic figures.

Immunohistochemistry:

S100 protein- Positive (only 50% cases)

Neuron-specific enolase, GFAP and neurofilament- Positive

Leu7 and myelin basic protein- weakly positive (not very reliable)

In the perineurial variant- S100 protein- Negative and EMA- Positive

In the epithelioid variant the number of S100 protein positive cells are much higher.

Heterologous elements show positivity with relevant immunostains.

Differential diagnosis:

Other sarcomas were ruled out mainly by immunochemistry.

The lack of expression of myogenic markers (desmin, caldesmon) rules out leiomyosarcoma.

Malignant melanoma is ruled out on the basis of negatic melanocytic markers other than S100 protein (HMB45, Melan-A, MART1).

Congenital and childhood plexiform (multinodular) cellular schwannoma. (Congenital and childhood plexiform (multinodular) cellular schwannoma: a troublesome mimic of malignant peripheral nerve sheath tumor. ).

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