**Abstract**

Benign skull base tumors include meningiomas, pituitary adenomas, craniopharyngiomas, and vestibular schwannomas. As an adjuvant therapy to surgery or when surgical treatment carries too high a risk of complications, a highly precise focused radiation, known as stereotactic radiosurgery or fractionated stereotactic radiation therapy, can be delivered to the tumor. The aim of this chapter is to systematically discuss benefits of the therapy, i.e., tumor control as well as complications and risk factors of the therapy relating to vision, hearing, hormone secreting regions, and cerebral vasculature. Meningiomas, pituitary adenomas, craniopharyngiomas, and vestibular schwannomas constitute the majority of primary skull base tumors amenable to stereotactic radiation therapy or radiosurgery and will be described in this chapter.

**Keywords:** skull base tumors, stereotactic radiosurgery, fractionated stereotactic radiotherapy, tumor control, vision, hearing, hormonal, stroke

## **1. Introduction**

Stereotactic radiosurgery (SRS) is defined as a single application of a high dose of radiation to a stereotactically precisely defined target [1, 2]. Stereotactic radiosurgery of the brain using the Gamma Knife or a Linear accelerator (LINAC) is a well-established and very effective therapy for brain metastases, arteriovenous malformations, and benign skull base tumors [1, 2]. The treatment utilizes differences in the biological sensitivity and repair capability of normal and pathologic tissue [3]. Stereotactic principles are used for calculating the radiation field. The patient wears a stereotactic head frame and undergoes a computed tomography (CT), which is subsequently fused with a preexisting magnetic resonance (MRI) scan, or an MRI is performed in the stereotactic head frame, the disadvantage being that there are often distortions of the magnetic field [4]. However, most lesions are better demonstrated on MRI scans. The aim of dose planning is to deliver a maximal dose to the tumor, while minimizing radiation dose to healthy brain structures. This is accomplished with conforming the radiation to the target and applying steep dose gradients [1, 2].

LINAC-based radiosurgery and radiation therapy devices accelerate electrons, and the electron beam is aimed at a heavy metal alloy target [1]. The resulting interactions between the electrons and the target produce photons, which can be collimated and focused on a patient. Multiple radiation beams are applied, each of which has its own entrance and exit points, while all are directed at the same target where they cross each other [1]. In LINAC radiosurgery and fractionated radiation

therapy, both the gantry and the treatment table rotate around the isocenter of the lesion for accurate delivery of the multiple beams [1]. The single radiosurgery radiation dose prescribed in LINAC-based radiosurgery for benign skull base tumors is commonly 10**–**17.5 Gy [1, 4–6]. Notably, in case of lesions adjacent to radiosensitive structures, fractionation is the preferred method of delivery, in which case different dose regimes apply [1, 7, 8]. In contrast to the Gamma Knife, LINAC offers the option of dose fractionation. Fractionated stereotactic radiation therapy (FSRT) utilizes the principles of conventional fractionation while taking advantage of stereotactic dosimetric techniques to conform the radiation to the tumor target. It is particularly suitable for treating skull base tumors, close to eloquent structures, such as the pituitary gland and optic nerves. A commonly used prescription dose for benign skull base tumors is a total of 54 Gy given with 1.8**–**2.0 Gy per fraction.

Radiosurgery with the Gamma Knife uses 201 separate cobalt sources, all aimed at a high dose at precisely one fixed target, with one or more isocenters employed, depending on the size and shape of the tumor [1–3]. A commonly used dose for benign skull base tumors is 12 Gy**–**16 Gy [3, 9].

Cyberknife is used in some centers and is a frameless robotic radiosurgery system, which is typically delivered in multiple session. It is a relatively safe and effective treatment for skull base tumors [10].

More recently, proton beam therapy has been introduced and is gaining progressively widespread use. It relies on protons produced end emitted by a synchrotron or cyclotron. The protons travel to a specific depth in the body depending on their energy and when striking the tumor rapidly emit their energy. It is well suited and used for various benign skull base tumors. Proton beam therapy is an effective treatment modality, with favorable long-term tumor control rates [11, 12].

The differences between Gamma Knife, LINAC and Cyberknife are summarized in **Table 1**.


#### **Table 1.**

*Differences between gamma knife, LINAC, and cyberknife.*
