Pattern and Rate of Tumor Growth

The reported growth rates of asymptomatic meningiomas are summarized in Table 7.1 . Olivero et al2 reported that 78% of 45 meningiomas followed up over a 2.5-year period did not grow, and the remaining 22% showed an increase in the mean maximum tumor diameter, with a growth rate of 0.24 cm/year. Herscovici et al3 reported that only one third of the meningiomas in their study grew during the follow-up period, and the mean annual growth was 3.2 mm. Yano et al4 found that 25 of 67(37.3%) asymptomatic meningiomas continued to grow during the 5-year follow-up period; their growth rate, calculated by the maximum tumor diameter, was 1.9 mm/year, and this finding was consistent with those of previous studies. In these studies, tumor diameter was used to quantify tumor size. Although this method is the easiest and most practical for measuring tumor growth and may be useful for determining treatment response, this method is not accurate.5,6 Because the shape of meningiomas, especially at the cranial bases, is complex, tumor growth in any direction that is not in line with the measuring axis can go undetected.7

Growth rate can be more accurately estimated by volu-metric analysis. Nakamura et al,7 who measured tumor volume on computed tomography and magnetic resonance imaging (MRI) and calculated the tumor-doubling time in 41 patients, found that the mean growth rate was 0.796 cm³/year (range 0.03 to 2.62 cm³/year) resulting in a 14.6% volume increase per year. At that rate, the mean tumor doubling time was 21.6 years. After conducting studies with 20 patients, including 13 symptomatic patients, Nakasu et al6 reported three meningioma growth patterns: Atypical meningiomas grew exponentially, whereas benign meningiomas grew exponentially, linearly, or not at all. More recently, Hashiba et al5 performed serial volumetric assessments throughout the follow-up period in patients with incidentally discovered meningiomas and investigated the growth patterns of these lesions by regression analysis. Results showed that 16 of the tumors grew exponentially and 15 grew linearly. The authors emphasized that in lesions that followed linear growth patterns, the yearly growth rates varied over time.

Braunstein and Vick8 hypothesized that meningiomas have variable growth rates, and that the growth rates reduce or even cease after the tumors attain a certain size. Another group also indicated that tumor doubling time and relative growth rates change during the life of the tumor.7 On the other hand, growth patterns of incidentally diagnosed meningiomas may fit the Gompertzian model, which describes exponential growth in the early stage, linear growth in the middle stage, and a plateau in size as tumor size increases.5,9

Although average tumor growth rates cannot be compared directly among the different studies because they employed different measurement methods, most asymptomatic meningiomas exhibited minimal growth.

Factors That Predict Tumor Growth

Several authors have identified various factors as predictors of tumor growth ( Table 7.2 ). Olivero et al2 showed that tumors in patients younger than 60 years of age did not grow more frequently or more rapidly than those found in patients older than 60 years of age, although the average follow-up time in this study was short, ~2.5 years. Nakamura et al7 also showed that in patients aged < 60 years, the absolute and relative tumor growth rates were higher and the tumor doubling time was shorter than in older patients. Herscovici et al3 indicated that older age was significantly associated with lower incidence of tumor growth and suggested that this relationship may involve aging-induced endocrine changes.

Yoneoka et al10 reported that tumor growth rates increase as tumor volume increases, and they suggested that patients with relatively large tumors should be observed carefully because the risk of tumor growth is high for these patients. Similar findings were reported in Niiro et al.11 On the other hand, Nakamura et al7 reported that initial tumor size cannot be considered as a predictive factor because tumor size shows only a moderate positive correlation with absolute annual growth rate and shows no correlation with tumor doubling time.

It has also been demonstrated that MRI hypointensity on T2-weighted images is associated with slowed tumor growth,1 and that MRI hyperintensity on T2-weighted images is associated significantly with faster tumor growth.7,11

There are no standard criteria for evaluating meningioma growth by radiology. The definition of tumor growth varies in the literature. For instance, in volume measurement studies, an annual growth rate of > 1 cm³/year10,6 or volume increases > 15%5 were considered tumor growth. In diameter measurement studies, growth has been defined as a change in tumor size of at least 2 mm,3 5 mm,12 or any measurable change.4 From these variations of definition, each study may yield very different predictive factors. However, even if these differences are taken into consideration, calcification may be the most significant predictable factor of slow tumor growth. Many authors have pointed out that meningiomas without calcification on imaging are more likely to progress than are calcified meningiomas,1,4–7,11,12 and no negative correlation between calcification and slow growth has been reported.

No significant differences in location have been reported between growing and nongrowing tumors. Whether a tumor was located at the skull base, or not, and whether it was located at the falx or parasagittal location did not appear to affect tumor growth ( Table 7.2 ).

Hashiba et al13 suggested that the proliferative potential of symptomatic meningiomas can be predicted using a noninvasive preoperative examination that considers the presence of peritumoral edema, ambiguous brain-tumor borders, and irregular tumor shape. However, the applicability of this method to predict the growth of asymptomatic meningiomas should be investigated.