Stereotactic Radiation Therapy

Stereotactic radiosurgery has been used worldwide as a primary or secondary procedure to treat both malignant and benign tumors. Typically, patients with brain metastases are treated following delivery of whole-brain irradiation.63,64 The clinical results including appropriate patient selection are discussed in detail in the chapter on Brain Metastases. The target (tumor) volumes for this procedure are small, typically less than 30 cm3 (less than 4cm diameter). To achieve this goal, intracranial stereotactic positioning systems are designed to achieve accurate target localization and have also enabled effective nonsurgical treatment of arteriovenous malformations, acoustic neuromas, and primary brain tumors, in addition to brain metastases.

Stereotactic radiation therapy to well-defined intracranial lesions can be performed in single or multiple fractions. The type of tumor can determine the choice of fractionation.63,64 Most centers perform radiosurgery on an outpatient basis. Treatment process starts with the fixation of a stereotactic frame on the patient's skull. Light sedation and local anesthetic applied before frame fixation are adequate for the patient's comfort throughout the treatment day. The patient with the frame goes through contrast-enhanced CT and/or MRI imaging with thin serial cuts (1-2 mm thick) taken in the region of the tumor. These scans are imported into a treatment planning software, where image segmentation is performed for volumetric evaluation and three-dimensional reconstruction.

There are two approaches for stereotactic radiosurgery: linear accelerator based and the gamma knife. With the first approach, tertiary micro-multileaf collimator and circular cones have been developed and adapted to a standard linear accelerator where 6MV photons are typically used. The cone sizes for linear-based stereotactic radiosurgery range from 0.5 to 3.5 cm. The second technical approach to stereotactic radiosurgery uses the gamma knife (Elekta). This machine uses 201 cobalt sources (60Co) and collimates the radiation beams using specially designed cranial helmets with 201 apertures and four different diameters (4, 8, 14, and 18 mm). The treatment is delivered with a combination of these helmets to conform the radiation distribution.

Whether a linear accelerator or a gamma knife is used, the radiation beams are arranged to intersect at a common point within the brain. The beam intersection volume is determined by the collimation system selected to encompass the target, and this produces a high-dose falloff just outside the intersection volume. Therefore, precise determination of the target volume is very important to deliver effective doses to the tumor(s). Noncoplanar arc beams are often used with linear accelerator-based stereotactic radiosurgery compared to stationary beams with the gamma knife.

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