What is a peripheral nerve sheath tumour (PNST)?

The term “peripheral nerve sheath tumour” (PNST) refers to a group of tumours that arise from the nerve sheath, which is the protective covering of the peripheral nerves. These nerves are the ones that extend outside the central nervous system (brain and spinal cord) to the rest of the body. The nerve sheath is made up of different types of cells, including Schwann cells, fibroblasts, and perineural cells, each of which can give rise to different types of tumours.

Common types of peripheral nerve sheath tumours

The most common types of peripheral nerve sheath tumours are:

• Schwannoma: A Schwannoma is a benign (noncancerous) tumour that arises from Schwann cells, which produce the myelin sheath that insulates nerve fibers.
• Neurofibroma: A neurofibroma is a benign (noncancerous) tumour made up of multiple types of cells including Schwann cells, fibroblasts, and perineural cells. This type of tumour is more common in people who have the genetic syndrome neurofibromatosis type 1.
• Malignant peripheral nerve sheath tumour (MPNST): An MPNST is an aggressive, cancerous tumour that arises from cells normally found in the nerve sheath. This type of cancer often develops from a pre-existing benign (noncancerous) neurofibroma in a patient with neurofibromatosis type 1, but it can also occur sporadically (in the absence of a genetic syndrome).

Microscopic appearance

The microscopic features of a peripheral nerve sheath tumour vary based on the specific type of tumour identified. For example:

• Schwannomas: Characterized by a well-defined capsule, their tissue is divided into two distinct areas: Antoni A and Antoni B areas. Antoni A areas are dense with cells and show palisading nuclei forming Verocay bodies, while Antoni B areas are less cellular, with a more myxoid and loose texture.
• Neurofibromas: Exhibit a mixture of cell types, including Schwann cells, fibroblasts, and perineural cells, embedded in a loose, myxoid stroma. They may not have a well-defined capsule, distinguishing them from schwannomas. When they arise in the skin, they often involve the dermis and subcutaneous tissue, appearing as a disorganized, fibrous mass.
• Malignant peripheral nerve sheath tumours (MPNSTs): These tumours display high cellularity, nuclear atypia (abnormal nuclear shapes and sizes), increased mitotic activity (dividing cells), and areas of necrosis (cell death). The cells may be spindle-shaped and arranged in haphazard patterns, often making them difficult to distinguish from other types of sarcomas without clinical and histological context.

Each type of peripheral nerve sheath tumour has distinct features, but there can be overlap, and immunohistochemistry (IHC) or molecular tests may be needed to confirm the diagnosis. These tumours’ appearance under the microscope helps pathologists determine their nature (benign vs. malignant) and the best treatment approach.