Nerve sheath tumors are growths developing from the protective coverings of nerves throughout the body, including the peripheral and, less commonly, central nervous systems. Their frequency varies based on general population data, tumor type, and genetic factors.
What Are Nerve Sheath Tumors?
Nerve sheath tumors originate from the protective layers surrounding nerve fibers, such as Schwann cells, when these cells grow uncontrollably and form a mass.
These growths can be benign (non-cancerous) or malignant (cancerous). The most common types are schwannomas, neurofibromas, and malignant peripheral nerve sheath tumors (MPNSTs). Over 90% of nerve sheath tumors are benign.
General Occurrence Rates
Nerve sheath tumors are relatively uncommon. Schwannomas, for instance, are rare, affecting fewer than 200,000 individuals in the United States annually.
Malignant peripheral nerve sheath tumors (MPNSTs) are even rarer, with an incidence of about 1 in 100,000 people annually. They represent 5% to 10% of all soft tissue sarcomas and are most frequently diagnosed in adults aged 20 to 50.
Most nerve sheath tumors are sporadic, occurring without a known genetic predisposition. However, they account for 24% to 31% of all primary spinal tumors, indicating their presence in specific anatomical locations despite overall rarity.
Prevalence of Specific Types
Among the various types of nerve sheath tumors, schwannomas are the most common form found in the peripheral nerves of adults. Although still rare, they typically occur as a single tumor in about 90% of cases. A significant proportion, around 60%, of benign schwannomas are vestibular schwannomas, also known as acoustic neuromas, which affect the nerve connecting the inner ear to the brain.
Vestibular schwannomas have an estimated incidence of about 1 case per 100,000 individuals per year. Spinal schwannomas are less common, with an incidence rate of approximately 0.24 per 100,000 persons between 2006 and 2014. These tumors commonly affect individuals between the ages of 50 and 60.
Neurofibromas involve several tissue types within the nerve sheath and can appear as nodules or masses, either under the skin or deeper within the body. Most neurofibromas are benign. Plexiform neurofibromas, a more complex type, are web-like growths that can extend into surrounding tissues and carry an 8% to 13% risk of transforming into malignant peripheral nerve sheath tumors.
Genetic Syndromes and Their Impact
Certain inherited genetic conditions increase an individual’s likelihood of developing multiple nerve sheath tumors.
Neurofibromatosis Type 1 (NF1) is the most common, affecting about 1 in 3,000 live births and accounting for approximately 96% of all neurofibromatosis cases. Individuals with NF1 frequently develop multiple neurofibromas, including plexiform neurofibromas, which are common in 40% to 60% of patients. The lifetime risk of developing a malignant peripheral nerve sheath tumor (MPNST) for individuals with NF1 is elevated, ranging from 8% to 13%. Up to 50% of all MPNSTs occur in patients with NF1.
Neurofibromatosis Type 2 (NF2) is less common than NF1, with a prevalence ranging from 1 in 33,000 to 1 in 87,410 births. Nearly all individuals with NF2 develop bilateral vestibular schwannomas, typically by age 30. This condition can also lead to other schwannomas and meningiomas, which are tumors of the brain and spinal cord coverings. Unlike NF1, MPNSTs are not generally a feature of NF2 in patients who have not received radiation therapy.
Schwannomatosis is the rarest of the three main genetic syndromes, with an incidence of about 0.58 cases per 1,000,000 people, or affecting approximately 1 in 40,000 individuals. This condition is characterized by the development of multiple schwannomas throughout the body. However, a distinguishing feature of schwannomatosis is that it typically does not involve the bilateral vestibular schwannomas seen in NF2. Around 15% of schwannomatosis cases are inherited.