**2. Epidemiology**

Meningiomas account for 20 and 38% of all primary intracranial tumors, respectively, in males and in females, with an incidence of about five cases per 100,000 persons [5]. In an autoptic series about 2–3% of the general population had an incidental asymptomatic meningioma [6]. The female sex and estrogenic conditions seem to be a risk factor for developing meningio‐ mas, with a female to male ratio of about 3:1 [7].

Hormonal therapy and hormone-dependent conditions such as breast cancer [8], pregnancy [9], or obesity [10] were in fact associated with a higher incidence of meningiomas. Cranial irradiation is also a recognized risk factor for developing meningiomas, usually with tumors having a more complex cariotype and a more aggressive behavior [11]. Head traumas have been for long associated with a higher meningioma incidence, but the causal effect was never demonstrated.

Some genetic conditions may favor the arising of meningiomas, such as neurofibromatosis type 2 (NF2), an autosomal dominant disorder characterized by mutation of the tumor suppressor gene NF2 coding for merlin in chromosome 22q12.2. Also multiple endocrine neoplasia type 1 (MEN1) is associated to a higher incidence of meningioma.

Meningiomas may derive from the dura of the cranial vault, of the skull base and at sites of dura reflection like the falx, the tentorium, and the dura recovering venous sinuses. Meningiomas may also arise from the optic-nerve sheath and from the choroid plexus. About 10% of them arise in the spine. In rare cases, meningiomas outside the craniospinal axis have been reported [12]. The preferential localization of intracranial meningiomas is summar‐ ized in **Figure 1**.

**Figure 1.** Graphic representation of the topographic distribution of intracranial meningiomas.

According to the 2007 World Health Organization (WHO) classification, meningiomas are divided in three categories: grade I or benign meningiomas, grade II or atypical meningio‐ mas, and grade III or anaplastic meningiomas [11]. With this new grading system, which includes the brain invasion into the diagnostic criteria for aggressiveness, the percentage of atypical meningiomas grew to 20–35% of newly diagnosed meningiomas [13]. This classifica‐ tion is important because, together with the extension of resection, it may help in predicting the recurrence rate and thus the global prognosis [14, 15] (**Figure 2**).

**Figure 2.** The recurrence rate at 5 years was stratified according to literature data: The recurrence after surgery is strict‐ ly related to the histologic grade of the lesion (WHO classification).
