Meningiomas with brain invasion alone show better survival than other grade 2 tumors

A newly published study from a team of neurosurgeons and neuropathologists is challenging a long-standing assumption in brain tumor classification, and the findings could reshape how patients with a common type of brain tumor are counseled and treated after surgery. The research, published in the Journal of Neuro-Oncology, examined a question that has divided the neuro-oncology community since 2016: when a meningioma—a tumor arising from the membranes covering the brain—is upgraded to a more dangerous grade solely because its cells have crept into brain tissue, is that tumor truly as threatening as other grade 2 meningiomas?

The answer, according to the data, appears to be no. Meningiomas classified as atypical—or World Health Organization grade 2—purely on the basis of brain invasion alone showed substantially better progression-free survival than atypical meningiomas that earned that designation through other histologic features. The distinction is not an academic curiosity. It touches on decisions about follow-up imaging schedules, adjuvant radiation therapy, and the level of anxiety patients carry home after leaving the operating room.

Meningiomas are among the most common primary brain tumors in adults, and most behave in an indolent fashion, growing slowly or not at all and often requiring nothing more than observation. The WHO classification system, which pathologists use to grade these tumors, underwent a pivotal change in 2016 when brain invasion—the microscopic presence of tumor cells infiltrating the brain parenchyma—was elevated to a major criterion that could single-handedly justify a grade 2 designation. Under the current system, a tumor with completely benign-appearing cells can still be labeled atypical if the pathologist identifies tumor nests threading into neural tissue. At the same time, grade 2 can also be assigned on the basis of mitotic activity, meaning the tumor is dividing more rapidly than normal, or on the basis of specific cellular features such as sheeting architecture, prominent nucleoli, small cell change, hypercellularity, or spontaneous necrosis.

The trouble, the study’s authors argue, is that brain invasion may not be the same biological signal as these other features. Tumors that grow large can, through sheer mechanical force and mass effect, press into and ultimately indent the brain, potentially allowing tumor cells to become entrapped along the irregular surface of the cortex. In that scenario, brain invasion could be a consequence of a tumor’s size and location rather than evidence of an intrinsically aggressive biology. A tumor that has simply been pushed against the brain, in other words, is not necessarily a tumor programmed to grow back.

To test that hypothesis, the researchers conducted a single-institution retrospective cohort study of patients who underwent meningioma resection between 2003 and 2023. The team meticulously reviewed pathology reports and regraded every tumor according to current WHO criteria, ensuring the analysis reflected contemporary diagnostic standards rather than shifting historical definitions. From this process, they isolated patients whose tumors fell into a category they designated BIOB—”Brain Invasion Otherwise Benign”—meaning these tumors were graded 2 solely because of brain invasion, with none of the other atypical features present. These patients were compared with the “other criteria” group, whose grade 2 designation rested on histologic characteristics other than invasion alone.

The cohort ultimately comprised 131 patients with a mean age of 59.2 years. The median preoperative tumor volume was a substantial 34.3 cubic centimeters, which is notable in itself—these were, on average, sizable tumors before surgery. Gross total resection, the neurosurgeon’s gold standard in which all visible tumor is removed, was achieved in 53 percent of cases. Fourteen percent of patients received adjuvant radiation therapy after their operations. Brain invasion was documented in 60 percent of the cohort, and of those, 45 patients—about a third of the total—formed the BIOB group. Mean radiographic follow-up stretched to 74 months, more than six years, giving the data genuine long-term weight.

The results were striking. Across the entire cohort, tumor progression occurred in 31 percent of patients, at an average of roughly 40 months after resection. But when the researchers split the cohort, the picture diverged sharply. Only 17.8 percent of BIOB patients—eight of forty-five—experienced progression, compared with 37.2 percent of patients in the other criteria group—thirty-two of eighty-six. In other words, patients whose tumors were graded upward solely because of brain invasion were less than half as likely to see their tumors return.

The statistical analysis reinforced what the raw numbers suggested. On univariate Cox regression, membership in the BIOB group was significantly associated with longer time to recurrence, with a hazard ratio of 0.44 and a p-value of 0.036. Gross total resection was also strongly protective, carrying a hazard ratio of 0.29. Critically, when the researchers adjusted for other variables in multivariate analysis, the BIOB designation remained a favorable independent predictor, confirming that the effect was not explained away by tumor size, extent of resection, patient age, or radiation treatment. Notably, the two groups were well matched at baseline: demographic, radiologic, and treatment characteristics did not differ significantly between them, with all comparisons yielding p-values above 0.3. The only hint of a difference was a trend toward younger age in the BIOB group—55.7 years versus 61.1 years—which fell just short of statistical significance.

The implications reach beyond meningioma nomenclature. In current practice, a grade 2 designation typically triggers more intensive surveillance MRI schedules, earlier consideration of radiotherapy for residual or recurrent disease, and a more cautious posture overall. Patients diagnosed with atypical meningiomas face the psychological burden of knowing their tumor is labeled aggressive. If a meaningful subset of grade 2 meningiomas—those defined by invasion alone—biologically resemble their benign counterparts, then these patients may be over-treated, over-monitored, and over-worried. Conversely, the study offers reassurance that the genuinely ominous features are the proliferative and cytologic hallmarks: rapid cell division, necrosis, and the constellation of atypical cellular changes.

The authors were careful to frame their findings within the study’s limitations. As a retrospective, single-institution analysis, it is vulnerable to selection bias and to the inherent variability in how pathologists sample and interpret brain invasion. Identifying invasion requires adequate tissue sampling at the tumor–brain interface, and under-sampling can miss invasion just as easily as over-enthusiastic interpretation can overcall it. The distinction between a tumor that mechanically compresses the brain and one that truly invades it can be genuinely difficult at the microscope, and interobserver variability among neuropathologists is well documented. These caveats have long been the central argument of skeptics who questioned whether brain invasion belonged in the grading system at all.

Yet the study’s methodological choices were designed to blunt those concerns. By regrading all tumors under current WHO criteria and by strictly separating tumors whose only grade-elevating feature was invasion from tumors with other atypical features, the researchers created a natural experiment: if invasion alone carried the same prognostic weight as the other criteria, progression rates in the BIOB group should have matched the other criteria group. Instead, they diverged dramatically, despite nearly identical baseline characteristics.

The mortality data add further texture to the story. Fifteen percent of the cohort died during follow-up, and among patients whose tumors did recur, a range of salvage treatments were deployed, including stereotactic radiosurgery in 18 percent, repeat surgical resection in 6 percent, fractionated stereotactic radiotherapy in 3 percent, and intensity-modulated radiation therapy in 2 percent. The mean tumor volume at the time of progression was 17.4 cubic centimeters, underscoring that even recurrent disease in this population was often detected and treated at manageable sizes—a point in favor of the surveillance protocols currently in place.

What emerges from this work is a compelling case for refining, rather than abandoning, the current grading framework. Brain invasion may still deserve attention as a descriptive finding—something a neuropathologist notes, a surgeon knows about, and a surveillance plan accounts for—without automatically consigning a patient to the full prognostic weight of an atypical diagnosis. Future studies incorporating molecular markers, which have increasingly demonstrated power to stratify meningioma risk beyond histology, may help clarify whether brain invasion interacts with tumor biology in ways the microscope alone cannot reveal. Until then, patients whose meningiomas are graded 2 on the basis of brain invasion alone now have data-driven grounds for a more optimistic conversation with their physicians.

For the broader neuro-oncology community, the study is a reminder that classification systems, however authoritative, are hypotheses about biology—and hypotheses must be tested against outcomes. When the outcomes part ways with the label, it is the label that should evolve.