How Fast Do Benign Schwannomas Typically Grow Each Year?

Benign schwannomas are noncancerous, generally slow-growing tumors, but their exact growth rate depends heavily on where they are located and how they are measured. For the most common intracranial type—a sporadic vestibular schwannoma (often called an acoustic neuroma)—the average growth in maximum diameter is approximately 1 millimeter (mm) per year ^[1][2]. However, group averages from medical studies do not predict how an individual tumor will behave. While active surveillance (a management strategy that monitors the tumor with regular MRI scans rather than immediate surgery or radiation) is a very common approach, patients frequently want to know exactly how fast these tumors grow. Data shows that many schwannomas experience no measurable growth during observation, while others grow steadily, and a very small percentage may even shrink spontaneously ^[3].

At-a-Glance: Reported Growth Rates by Location

Growth Rates in Detail

Evidence on growth rates is strongest for vestibular schwannomas. Data for spinal and peripheral schwannomas is much more limited, often relies on smaller observational studies, and is usually measured in volume (total 3D mass) rather than a straight line ^[7][6].

Vestibular Schwannomas (Acoustic Neuromas)

Because these are heavily researched, doctors have clear data on how they behave over time:

• Average Linear Growth: The typical growth rate ranges from 0.4 mm to 2.9 mm per year, averaging out to about 1 mm to 1.1 mm annually ^[1][4].
• If It Starts Growing: Tumors that have already shown growth on a previous scan tend to grow slightly faster in the future, averaging around 3 mm per year ^[1].
• Stability and Shrinkage: Over a given observation period, roughly 41% to 51% of vestibular schwannomas will show measurable growth, while 46% to 59% will show no measurable growth (remaining stable under that study’s criteria) ^[3][5]. Surprisingly, around 3% (and up to 13% in studies using sensitive 3D volume measurements) actually shrink on their own without any treatment ^[3][8].
• The “Time Factor”: The longer a tumor stays stable, the less likely it is to grow. In one observational cohort, the chance of a stable tumor growing in the next year dropped from 21% in the first year to just 2% by year five ^[5]. However, this is a lower risk, not a zero risk, so continuing scheduled surveillance is essential. Furthermore, nearly 18% of actively growing tumors may eventually experience spontaneous growth arrest (where a growing tumor stops expanding during the observation period) ^[9].

Spinal Schwannomas

For schwannomas located inside the spinal canal, specialists often measure the tumor’s overall volume (a percentage increase) rather than its diameter. This means you cannot directly compare spinal rates to the 1 mm/year seen in vestibular tumors.

• In one cohort study of spinal schwannomas undergoing surveillance, the median volumetric growth rate was about 5.5% per year ^[6].
• When looking at subgroups within that study, the tumors that actively grew increased in volume by a median of 13% per year, while the stable ones remained at near 0% growth ^[6].

Peripheral Schwannomas

Peripheral schwannomas grow on the nerves in the limbs (arms and legs) or other areas of the body. Data here is limited to specific subgroups:

• Distal peripheral nerves (limbs): A single study reviewing these specific tumors found distinct volume-based growth patterns: about 58% followed a slow-growth pattern (increasing in volume by roughly 5% to 10% per year) ^[7]. Conversely, about 41.5% showed a faster growth pattern, increasing their volume by over 80% per year ^[7]. (Note: An 80% volume increase does not mean the tumor is 80% wider; a small increase in diameter can lead to a large increase in 3D volume).

Linear vs. Volumetric Measurement: Why It Matters

When discussing growth rates with your doctor, it is important to know how the tumor is being measured, as the two methods are not interchangeable:

• Linear Measurement (Maximum Diameter): This measures the longest straight line across the tumor in millimeters.
• Volumetric Measurement (3D Volume): This calculates the total volume of the tumor. Volumetric imaging detects growth much earlier than linear measurements ^[10]. Depending on the tumor’s starting shape and size, it might increase its total volume by 20% before its maximum diameter grows by a mere 2 millimeters ^[10].

Apparent small changes in size can sometimes simply be measurement noise (minor variations caused by differences in MRI machines, slice thickness, or how the radiologist read the scan). Clinicians generally look for a reproducible trend across multiple scans rather than reacting to one isolated number.

Symptoms Matter as Much as Size

Treatment decisions are never based on a single growth threshold. Doctors also evaluate your age, overall health, and whether the tumor is pressing on critical structures. You should contact your care team promptly if you experience new or changing symptoms, even if your recent MRI showed the tumor was stable:

• For Vestibular Schwannomas: Sudden hearing loss, new or worsening facial numbness/weakness, or severe balance issues.
• For Spinal Schwannomas: New weakness or numbness in the arms or legs, difficulty walking, or new bowel/bladder incontinence.
• For Peripheral Schwannomas: Rapidly worsening pain, a noticeably enlarging mass, or loss of function in the affected limb.

(Note: Sudden or acute neurological symptoms, like sudden hearing loss or loss of bowel control, require urgent medical assessment).

Patients with NF2-related schwannomatosis (or other genetic tumor-predisposition syndromes) often have multiple tumors that behave differently than solitary, sporadic schwannomas. These conditions require an individualized, specialist-directed surveillance plan ^[11][12].