“You can observe a lot by just watching” (Yogi Berra)

Hospital-based intracranial tumor series demonstrate that the incidence of meningiomas is approximately 20% of all intracranial tumors and therefore the most common nonglial primary intracranial tumor whereas autopsy-based studies find an overall incidence of 30%.¹ In unselected patient populations, the incidence of incidentally and asymptomatic discovered meningiomas by MRI is approximately 1%. Furthermore, 2% of all autopsies reveal incidental and undisclosed meningiomas. There is an age-dependent incidence of meningiomas with 0.3 cases per 100 000 children in childhood and 8.4 per 100 000 individuals in the elderly.¹ Intracranial meningiomas are most often discovered in adults in their fourth, fifth, and sixth decades of life and are rare in childhood (only 2% of all meningiomas occur in childhood). Meningiomas are more common in blacks as compared to Caucasians, the reverse of that observed for gliomas. Additionally, meningiomas are more common in females with a 2:1 female to male ratio in intracranial meningiomas.

Pathologically, 95% of meningiomas are benign (WHO grade 1), 6%-8% are atypical (WHO grade 2) and 1%-2% are malignant (WHO grade 3). Most patients diagnosed with a symptomatic meningioma undergo surgical resection to relieve neurological symptoms such as headache or focal neurologic deficits. Complete surgical resection can be curative determined both by the Simpson scale extent of resection and the underlying WHO grade. In instances of an incompletely resected or recurrent meningiomas not previously irradiated, radiotherapy is administered. Radiotherapy may be administered as conventional external beam irradiation, conformal radiotherapy, or stereotactically by linear accelerator (LINAC), Leksell Gamma Knife, or Cyberknife radiosurgery. Advocates of stereotactic radiotherapy (SRS; either single fraction or multifractionated) have proposed this therapy in lieu of surgery particularly in poor surgical risk patients, patients with meningiomas in eloquent or surgically inaccessible locations, and in patients of advanced age.

The clinical presentation of meningiomas, as with all intracranial mass lesions is dependent upon location in the brain. 90% of meningiomas are found in the supratentorial compartment. Generally, meningiomas are slow-growing tumors, and symptoms at presentation are rarely precipitous, but more often chronic and insidious in nature. New onset and slowly evolving headaches are very common and usually unassociated with symptoms suggestive of raised intracranial pressure which reflects the slow growth of these tumors. A history of long-standing partial seizures is not uncommon for convexity meningiomas nor is an insidious personality change (often confused with dementia or depression) in patients with large inferior frontal meningiomas.

Several studies have examined the growth rate of incidental meningiomas, meningiomas discovered in an otherwise asymptomatic patient.^2–7 Overall, these studies conclude that most asymptomatic meningiomas may be followed safely with serial brain imaging until either the tumor enlarges significantly or becomes symptomatic. In addition, these studies confirm the observation that many meningiomas grow very slowly (on average 3-4 mm in maximum dimension/year) and that a decision not to operate is justified in selected asymptomatic patients. In that the growth rate is unpredictable in any individual, repeat serial brain imaging is required to monitor an incidentally discovered asymptomatic meningioma. A recent observational study using serial volumetric assessment conclude meningiomas manifested 3 types of growth patterns, such as no growth, linear (mean annual volumetric growth of 15%), and exponential (mean annual volumetric growth of 25%) growth.⁷ Repeat contrast MRI 3 months after the initial documentation of an asymptomatic meningioma, followed by scans at increasing intervals (ie, every 6 months, once per year, every other year) as stability is confirmed over the first 1-2 years appears adequate to assess growth rate and the need for intervention as suggested by multiple guidelines.

Sheehan et al report on a multicenter matched cohort analysis of incidental meningioma followed for imaging-based progression during active surveillance or after SRS.⁸ The main study endpoint is imaging-based disease control. The principal for a treatment policy of observation is to ensure quality time without an intervention when such an intervention may not be needed. The question, therefore, is whether the balance of risk and benefit of offering treatment is superior to observation especially when disease progression may be associated with little or no risk of neurological deficit. Therefore, the most relevant outcome of importance is neurological progression-free survival or survival free of a decline in quality of life, neither of which were assessed in the current study. The authors conclude all asymptomatic newly diagnosed solitary and presumed WHO grade I meningioma be considered for treatment with SRS. The issues with such a broad conclusion are discussed by the authors but perhaps overshadowed by the recommendation for early treatment with SRS. In that this was a retrospective study without clear reasons for treatment assignment (SRS vs observation) there are likely considerable confounders and bias. It is unclear whether the patients were considered for surgery (the only curative therapy) or rather were all tumors in a surgically inaccessible location. Further, there was no prespecified statistical analysis such that the comparison between propensity matching and overall population enrolled is subject to bias. Additionally, the definition of a new neurological deficit is never defined and consequently, the meaning of a new neurological deficit is unclear. The choice of surveillance imaging schedule varied by institution and this variable would impact results. Likely, there was a difference in the frequency with which treated and untreated meningiomas were imaged in surveillance. The cost of treatment, ie, the economic impact of such a recommendation (treating all asymptomatic unifocal meningiomas with SRS) would be enormous (given the high prevalence in the population) and is not discussed. There is no data provided that suggests a critical tumor size for which early treatment may be warranted whereby an analysis stratifying tumor by size and outcome might be informative in determining a threshold tumor size warranting early treatment. The argument for early treatment with SRS harkens to that of treating low-grade glioma with early or delayed radiotherapy. A randomized clinical trial is warranted to answer this question of the value of early vs delayed treatment in the management of incidental and asymptomatic meningioma that would include as study endpoints neurologic and RANO-based progression-free survival.

Conflict of interest statement. None declared.