Background

Accounting for 15% of all meningiomas, convexity meningiomas are the most common tumors of the cranial vault.1 The term convexity refers to those meningiomas that do not extend into the dura mater of the skull base and do not involve the dural venous sinuses. As opposed to tumors in other locations, these lesions are often readily accessible. Advancements in diagnostic imaging (leading to earlier diagnosis) and surgical technology have cumulatively resulted in improved surgical outcomes, with modern studies indicating lower morbidity and mortality rates (complication rate 1.7 to 9.4% and 30-day mortality of 0%) in comparison with other anatomical locations.2,3 The difficulties often lie in deciding when to operate and how to manage recurrent or residual disease. The emergence of stereotactic radiosurgery as a viable treatment option for smaller meningiomas not only has altered the patient population undergoing microsurgical resection but also has altered the goals of resection, the management of recurrent disease, and the expectations of the patient. Unfortunately and surprisingly, there are relatively few modern studies analyzing long-term outcomes of patients with convexity meningiomas that can aid in this decision-making process. Data are often extrapolated from studies assessing various other anatomical sites. This chapter discusses the epidemiology, location, presentation, radiographic findings, indications for treatment, and operative management of these lesions.

Epidemiology and Location

Convexity meningiomas account for nearly 15% of all meningiomas (second-highest incidence after parasagittal meningiomas).

Originally, convexity meningiomas were classified by site: frontal, paracentral, parietal, occipital, and temporal. In 1938, in Meningiomas: Their Classification, Regional Behavior, Life History and Surgical End Results, Cushing and Eisenhardt incorporated the following stratification: precoronal, coronal, postcoronal, paracentral, parietal, occipital, and temporal. This subclassification is advantageous in that it recognizes the eloquent (or noneloquent) nature of the underlying cortex. To this categorization, one can also add pterional or lateral sphenoid wing meningiomas that grow solely or mainly outward toward the frontal and temporal lobes.

Convexity meningiomas can further be classified on radiographic appearance: globose or en plaque. Globose refers to the classic spherical, lobulated mass, whereas en plaque meningiomas are those lesions that have a flatter, carpetlike appearance infiltrating the dura.

Presentation

Considering that meningiomas are generally slow-growing tumors and that the underlying cortex can be non-eloquent, patients with convexity meningiomas typically have a protracted clinical course and larger-than-expected tumors at the time of diagnosis (if they are diagnosed due to the onset of symptomatology). However, due to the increased accessibility and safety of obtaining imaging (diminished risk of radiation with magnetic resonance imaging as opposed to computed topography), an increasing number of asymptomatic lesions are being discovered (14 to 20%).2,3 In these situations, the reasons for imaging can range from history of carcinoma or lymphoma to sinusitis.2 The increased incidence of detection of a meningioma in an asymptomatic patient has placed a renewed focus both on the natural history of meningiomas and on the importance of identifying firm indications to offer treatment (see the following discussion).

In the “symptomatic” patient population, most patients present with signs and symptoms attributable to mass effect and the tumor site. Headache, by far, is the most common symptom, occurring in anywhere from 39 to 48% of this patient population.2,3 A history of seizures (20 to 34%) remains common in modern-day studies and is most often seen with temporal meningiomas.2,3 Other site-specific symptoms can include confusion, memory loss, depression/personality alteration (frontal); motor or sensory deficit (perirolandic lesions); visual field deficits, including field cuts, disturbances in color perception and tracking objects (occipital); neglect, alexia, and difficulty with calculation (parietal); aphasias (dominant-sided pterional or posterior temporal lesions).

Radiographic Findings/Diagnostic Aids

Tremendous improvements in modern radiographic techniques have resulted in increasing the amount of information that can be gleaned preoperatively to help with surgical planning. Magnetic resonance imaging is the mainstay not only of diagnosis but also of surgical planning. On T1-weighted magnetic resonance, ~60% of meningiomas are isointense, whereas 30% are hypointense compared with gray matter. As with computed tomography (CT), meningiomas display strong enhancement with contrast administration; in addition, enhancement of the surrounding dural margin may be noted (“dural tail”). The significance of this dural tail has been controversial in the surgical management of these tumors. Many believe this to represent neoplastic infiltration of surrounding dura (and hence requiring resection), whereas others feel this represents more of a reactive inflammatory response. Correlative radiographic-histopathologic studies have demonstrated that in 65% of patients this dural tail represents tumor invasion, whereas in the remainder of patients only nonneoplastic meningothelial proliferation, hypervascularity, and vascular dilatation were seen.4

On T2-weighted magnetic resonance imaging (MRI), several important radiographic findings can be appreciated preoperatively. Information regarding the extent of demarcation between tumor and surrounding brain can be noted by carefully assessing for a “cerebrospinal fluid (CSF) cleft” around the lesion. T2-weighted imaging also allows an easier determination of the possibility of adjacent brain invasion, which can be evidenced by T2 hyper-intensity (edema), and of the local vasculature, identified by vascular flow-voids. T2-weighted imaging can often foreshadow the nature of the tumor’s consistency that may be encountered intraoperatively. Hyperintensity on T2-weighted imaging suggests a higher water content indicating a softer tumor, whereas hypointensity can indicate a fibrous or calcified tumor requiring different surgical considerations.

The extent of peritumoral edema can also be analyzed on T2-weighted imaging. The extent of edema is often an indication for treatment intervention. In addition, the presence of peritumoral edema can be associated with particular radiological and histological features affecting surgical treatment.5 Most often, meningothelial, ana-plastic, microcystic, and angiomatous subtypes display higher edema indices than other types. However, more importantly, studies have demonstrated two significant radiographic factors to be associated with peritumoral edema: vascular supply from pial-cortical arteries and cortical invasion (absence of a well demarcated T2 plane on MRI).

Although we do not typically obtain magnetic resonance angiography or conventional angiography for most convexity meningiomas, it can serve as an adjunct in the preoperative assessment of some tumors. It enables the surgeon to assess the extent and pattern of vascularity, the extent of tumor encroachment on vascular structures, and the feasibility of embolization. Occasional irregular patterns of vascularity can be discovered, such as vascular supply from the contralateral middle meningeal artery ( Fig. 15.1 ).

Indications for Treatment

The decision to treat convexity meningiomas can be complex and must take into account several factors. The options of observation, surgery, or radiation are all relevant. The natural history or potential growth rate must be considered.6,7 A recent article by Yano and Kuratsu demonstrated that 37% of meningiomas showed growth on imaging during observation for 3.9 years.7 Furthermore, additional consideration is necessary based on the fact that the growth rate of meningiomas might be linear and not logarithmic (as seen with malignant astrocytomas).6,7 However, in our experience, the growth of observed lesions can often be erratic (or intermittent) where lesions that have been stable on serial imaging can demonstrate relatively rapid growth within a short time period. The last issue to consider in deciding upon a period of observation is that higher-grade meningiomas can be missed, with subsequent delay in surgery resulting in potentially higher operative risk and possibly affecting the long-term outcome.

Although there have been several attempts in the literature to develop a decision-making algorithm for meningiomas (i.e., the “ABC” system and “CLASS” algorithm), these systems apply to cranial base meningiomas.8 Our decision to treat is primarily based on the patient’s age/medical status, tumor size, symptom complex, and associated edema. We have adopted an increasingly aggressive treatment philosophy considering the favorable long-term outcomes from convexity meningioma surgery—0% surgical mortality, ~3 to 5% morbidity (neurological and nonneurological), and 0% recurrence with grade zero resection and World Health Organization (WHO) grade I meningiomas.2,3 We will typically observe lesions in asymptomatic patients who are over the age of 70 or have poor medical status and do not have any associated peritumoral edema. In younger patients who are asymptomatic, with tumors smaller than 3 cm, and who do not have associated edema, we discuss the surgical and the conservative options and let the individual patient decide after presenting a detailed outline of the risks and benefits of each plan. We do have several patients who have been followed vigilantly, and only a small fraction of them have required subsequent surgery (without negative repercussions related to a “delayed” surgical treatment).

Otherwise, in most of the other patients we will recommend surgical intervention for diagnosis and complete resection. We are reluctant to recommend radiosurgery as the initial treatment for any tumor where histological grade is not known for fear of improperly treating WHO grade II and III meningiomas, where a combined surgical and radiation therapy approach is preferable.