**2.1 Meningiomas**

Meningiomas are the most common primary intracranial tumors, the prevalence being approximately 100 per 100,000 [13, 14]. They are slow-growing tumors, most often benign and dural-based. Meningiomas are classified according to the World Health Organization (WHO) classification of grade, where grade I is benign, grade II atypical, and grade III anaplastic [13, 15–17]. Approximately 95% of intracranial meningiomas are benign and approximately 5% are atypical or anaplastic [13, 15]. Atypical and anaplastic meningiomas have an increased recurrence and mortality risk [15, 16]. In addition to WHO grade, prognosis and recurrence risk depend on the radicality of resection [13, 18]. Anterior skull base meningiomas are defined as arising anterior to the chiasmatic sulcus, which separates the middle and the anterior cranial fossa. Anterior skull base meningiomas include olfactory groove, tuberculum sellae, sphenoid wing, cavernous sinus, and optic nerve sheath meningiomas [19, 20]. Medial skull base meningiomas include clival and petroclival meningiomas [21]. Olfactory groove meningiomas arise from the cribriform plate in the midline and often compress or distort the olfactory and optic nerves and optic chiasm (**Figure 1**). Tuberculum sellae meningiomas are usually located in the suprasellar and subchiasmal region in the midline and often compress the optic nerves and internal carotid arteries (**Figure 2**). Sphenoid wing meningiomas arise from the sphenoid wing and often involve the optic nerves, the cavernous sinus, or carotid arteries, and cause neurological damage by direct compression of adjacent cranial nerves (**Figure 3**). Cavernous sinus meningiomas may either originate within the cavernous sinus and spread outside of it or originate outside the cavernous sinus and invade it. Cavernous sinus meningiomas often present with symptoms related to compression of structures within the cavernous sinus, resulting in ophthalmoplegia or facial pain or numbness or ischemic stroke due to compression of the carotid artery and with tumor extending beyond the cavernous sinus, can also affect the optic nerves and chiasm or the pituitary gland (**Figure 4**). Total resection is often not possible, and resection is also associated with risks to the carotid artery, or damage to the cranial nerves of the cavernous sinus [22]. Optic nerve sheath meningiomas are rare, accounting for 1–2% of intracranial meningiomas, and due to their localization, management is often conservative. Finally, clival and petroclival meningiomas arise from the clivus and typically compress the brain stem, and they often involve the cavernous sinus and are surgically particularly challenging (**Figure 5**) [21].

With incompletely resected or recurrent skull base meningiomas, stereotactic radiation therapy or radiosurgery is recommended [13, 23]. Also, the extent of surgical tumor removal is dependent on tumor's localization adjacent to critical structures. Surgical treatment of cavernous sinus meningiomas, in particular, is associated with a high risk of cranial nerve injury, especially ophthalmoplegia, and therefore a high proportion of cavernous sinus meningiomas are treated by stereotactic radiation or radiosurgery and in some institutions is the first-line treatment. Generally, stereotactic radiosurgery or fractionated radiation therapy is frequently used as primary therapy in surgically high-risk tumors, resulting in good local control [4, 10, 13, 23, 24].

**Figure 1.** *MRI scan with gadolinium (Gd) of an olfactory groove meningioma.*

#### **Figure 2.**

*MRI scan with Gd of a tuberculum sellae meningioma.*
