A meningioma is the most common form of primary brain tumor, arising from the meninges, the protective layers of tissue surrounding the brain and spinal cord. Most of these tumors are classified as benign, meaning they are non-cancerous and typically grow slowly. Because of this slow growth, many meningiomas are discovered incidentally during imaging for an unrelated condition, often without causing any symptoms. The speed at which a meningioma grows is highly variable, ranging from undetectable change over years to rapid expansion, which is why monitoring the growth rate is so important for long-term management.
Understanding the Typical Growth Rate
The vast majority of meningiomas are classified as World Health Organization (WHO) Grade I, which is associated with a very slow rate of growth. For these typical benign tumors, the average linear growth rate is often cited as between 1 and 4 millimeters per year. Many small, asymptomatic tumors remain stable, showing no measurable growth over several years of observation.
A more precise metric is the tumor doubling time, which measures the time required for a tumor’s volume to double in size. For slow-growing meningiomas, the median tumor doubling time can span decades. This slow speed is why a “watchful waiting” approach is often recommended for tumors that are small and not causing symptoms. The growth dynamic is not always linear, however, and some studies suggest meningiomas follow a Gompertzian pattern, meaning their growth rate may slow down as they get larger.
Factors That Accelerate or Slow Tumor Growth
The speed of meningioma growth is influenced by its biological classification, defined by the World Health Organization (WHO) grading system. Grade I tumors are the most common, making up about 80% of cases, and are the slowest growing. Tumors classified as Grade II (atypical) and Grade III (anaplastic or malignant) are much rarer but demonstrate significantly faster growth and a higher rate of recurrence.
Grade II meningiomas represent about 15% to 18% of cases and are considered intermediately aggressive, growing faster than Grade I tumors. Grade III tumors, which account for only 1% to 4% of cases, are malignant and grow rapidly, often invading surrounding brain tissue. The presence of specific genetic changes, such as homozygous deletions of the CDKN2A/B genes, is also associated with a more aggressive, Grade III behavior.
Hormonal factors can also contribute to variations in growth speed, particularly in women. A high percentage of meningiomas, around 70%, express progesterone receptors. This hormonal sensitivity can lead to increased tumor growth during periods of elevated progesterone, such as in the second and third trimesters of pregnancy. Changes in tumor size have also been observed during the menstrual cycle, suggesting a clear biological link between sex hormones and growth dynamics.
How Doctors Monitor Growth Over Time
For meningiomas that are small and asymptomatic, doctors typically recommend an approach known as active surveillance or “watchful waiting.” The primary tool for monitoring growth is magnetic resonance imaging (MRI) or, less often, computed tomography (CT) scans. These imaging studies allow physicians to accurately measure the tumor’s size and compare it over time.
The schedule for monitoring varies based on the tumor’s initial appearance and the physician’s assessment of its risk. Initially, a follow-up scan might be scheduled within six months to determine a baseline growth rate. If the tumor is stable, the interval between scans is often extended to annually. Radiologists measure changes in linear dimensions or use volumetric analysis to calculate the precise change in tumor size.
When Growth Rate Dictates Treatment
The decision to move from observation to active treatment, such as surgery or radiation, is heavily influenced by the measured growth rate and the appearance of symptoms. Treatment is generally reserved for tumors that are causing neurological symptoms, regardless of their size, or for tumors that demonstrate aggressive growth. A rapid volumetric increase is a significant trigger for intervention, indicating the tumor may be biologically more aggressive than initially suspected.
While a linear growth rate of a few millimeters per year is considered slow, a substantially faster rate suggests the need for action. For example, a volumetric increase of 40% over a six to twelve-month period has been shown to correlate with a worse prognosis for some meningiomas. Furthermore, if a tumor’s growth pushes it into a location where it begins to compress critical brain structures, treatment will be recommended to prevent permanent disability.