The precise natural history of incidentally discovered meningiomas (IDMs) remains unknown. It has been reported that for symptomatic meningiomas, tumor location can be used to predict growth. As to whether the same is true for IDMs has not been reported. This study aims to answer this question and provide biological evidence for this assumption by extending the study to involve symptomatic cases.

A total of 113 IDMs were analyzed by fine volumetry. A comparison of growth rates and patterns between skull base and non-skull base IDMs was made. Subsequently, materials obtained from 210 patients with symptomatic meningiomas who were treated in the authors' hospital during the same period were included for a biological comparison between skull base and non-skull base tumors using the MIB-1 index.

The 110 patients with IDMs included 93 females and 17 males, with a mean follow-up period of 46.9 months. There were 38 skull base (34%) and 75 non-skull base (66%) meningiomas. Forty-two (37%) did not exhibit growth of more than 15% of the volume, whereas 71 (63%) showed growth. Only 15 (39.5%) of 38 skull base meningiomas showed growth, whereas 56 (74.7%) of 75 non-skull base meningiomas showed growth (p = 0.0004). In the 71 IDMs (15 skull base and 56 non-skull base), there was no statistical difference between the 2 groups in terms of mean age, sex, follow-up period, or initial tumor volume. However, the percentage of growth (p = 0.002) was significantly lower and the doubling time (p = 0.008) was significantly higher in the skull base than in the non-skull base tumor group. In subsequently analyzed materials from 94 skull base and 116 non-skull base symptomatic meningiomas, the mean MIB-1 index for skull base tumors was markedly low (2.09%), compared with that for non-skull base tumors (2.74%; p = 0.013).

Skull base IDMs tend not to grow, which is different from non-skull base tumors. Even when IDMs grow, the rate of growth is significantly lower than that of non-skull base tumors. The same conclusion with regard to biological behavior was confirmed in symptomatic cases based on MIB-1 index analyses. The authors' findings may impact the understanding of the natural history of IDMs, as well as strategies for management and treatment of IDMs and symptomatic meningiomas.