Malignant nerve sheath tumours

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Malignant nerve sheath tumours, also known as MPNST (Malignant Peripheral Nerve Sheath Tumours), are rare soft tissue sarcomas originating from peripheral nerves or cells associated with nerve sheaths. As they may originate from different cell types, sometimes it may be particularly difficult to diagnose them.

Also called malignant schwannomas, malignant peripheral nerve sheath tumours are often particularly aggressive, tend to recur or metastasize, and can affect several anatomical sites. In most cases, MPNST originate in the main trunk nerves such as the sciatic nerve or the brachial and sacral plexuses; in other cases, they occur at the head and neck level, and near the limbs. They may arise spontaneously or develop from an already existing neurofibroma.

Malignant nerve sheath tumours, which account for around 5%-10% of all soft tissue sarcomas, have an estimated incidence of 1: 100,000. They can affect both men and women equally, with an average age of onset between 20 and 50 years in general.

Causes of malignant nerve sheath tumours

The causes of malignant nerve sheath tumours are still unknown. Some of the risk factors of these tumour forms include:

• Massive exposure to radiation, for instance, in previous radiation therapies
• Presence of other cases in the family

It has also been shown that some diseases can increase the risk of developing a malignant nerve sheath tumour, such as:

• Neurofibromatosis type 1
• Gardner syndrome
• Li-Fraumeni syndrome
• Retinoblastoma

In about 50% of cases MPNST appear in patients with neurofibromatosis type 1 (NF1).

Symptoms of malignant nerve sheath tumours

Malignant nerve sheath tumours can originate from different types of cells, so symptoms may differ from case to case. In many cases, patients complain of:

• Pain
• Limb swelling
• Joint pain
• Paresthesia
• Hyposthenia
• Neurological deficits

Diagnosis of malignant nerve sheath tumours

Diagnosing this type of tumour is sometimes difficult due to the differentiated origin of the tumour. After visiting a specialist in order to define the patient’s medical history, diagnostic tests must be carried out to locate the tumour and evaluate its extent. 
In particular:

• Computed tomography (CT)
• Nuclear magnetic resonance (NMR)
• A positron emission tomography (PET) scan

A histological test is required to confirm the diagnosis, although this may be very complicated due to the absence of specific immunohistochemical or molecular markers.

A biopsy also allows to define the staging of the tumour, which is crucial to plan the correct therapy and obtain prognostic indications. In the case of soft tissue sarcomas, staging depends on:

• Histological grade
• Size of tumour
• Depth of location
• Presence of metastasis

For this type of cancer, the staging method of the American Joint Committee on Cancer (AJCC) is used:

• Stage I: small low-grade lesions, with no evidence of metastasis
• Stage II: high-grade tumours, size smaller than 5 cm or larger than 5 cm but more superficial; no metastasis in both cases
• Stage III: high-grade, deep tumours, size larger than 5 cm; no metastasis
• Stage VI: any lesion with presence of distant metastases

Following the diagnosis, the areas that are more likely to be affected by metastasis from malignant nerve sheath tumours must be checked, that is:

• Lungs
• Bones
• Pleura

Treatment of malignant nerve sheath tumours

The treatment of choice is surgical resection, usually combined with radiotherapy. In cases where surgery is not feasible, radiotherapy is the treatment of choice.