Radiosurgery for Metastatic Brain Tumors

Introduction

Stereotactic radiosurgery (SRS) is a minimally invasive treatment modality that delivers a large and typically single dose of radiation to a specific intracranial target while sparing surrounding tissue. Unlike conventional fractionated radiotherapy, SRS does not totally rely on the higher radiosensitivity of neoplastic lesions relative to normal brain (therapeutic ratio). Its selective destruction is dependent mainly on sharply focused high-dose radiation and a steep dose gradient away from the defined target. The biologic effect is irreparable cellular damage and delayed vascular occlusion within the high-dose target volume. Because a therapeutic ratio is not required, traditionally radioresistant lesions can be treated. However, because destructive doses are used, any normal structure included in the target volume is subject to damage.

The basis for SRS was conceived more than 40 years ago by Lars Leksell. He proposed the technique of focusing multiple nonparallel beams of external radiation on a stereotactically defined intracranial target. The averaging of these intersecting beams results in very high doses of radiation to the target volume but innocuously low doses to nontarget tissues along the path of any given beam. His team’s implementation of this concept culminated in the development of the Gamma Knife. The patient is stereotactically positioned in the Gamma Knife so that the intracranial target coincides with the isocenter of radiation. Using variable collimation, beam blocking, and multiple isocenters, the radiation target volume is shaped to conform to the intracranial target.

An alternate radiosurgical solution using a linear accelerator (LINAC) was first described in 1984 by Betti et al. Colombo et al. described such a system in 1985, and LINACs have subsequently been modified in various ways to achieve the precision and accuracy required for radiosurgical applications. In 1986 a team composed of neurosurgeons, radiation physicists, and computer programmers began development of the University of Florida LINAC-based radiosurgery system. This system has been used to treat more than 4500 patients at the University of Florida since May 1988, and it is in use at multiple sites worldwide.

Most LINAC radiosurgical systems rely on the same basic paradigm: A collimated x-ray beam is focused on a stereotactically identified intracranial target. The gantry of the LINAC rotates around the patient, producing an arc of radiation focused on the target ( Fig. 88.1 ). The patient couch is then rotated in the horizontal plane and another arc performed. In this manner, multiple non-coplanar arcs of radiation intersect at the target volume and produce a high target dose, with minimal radiation to surrounding brain. This dose concentration method is exactly analogous to the multiple intersecting beams of cobalt radiation in the Gamma Knife ( Fig. 88.2 ).

The target dose distribution can be tailored by varying collimator sizes, eliminating undesirable arcs, manipulating arc angles, using multiple isocenters, and differentially weighting the isocenters. In recent years, a number of LINAC systems have used alternative beam-shaping techniques involving “intensity modulation” and micromultileaf collimators (discussed under dose planning). Achievable dose distributions are similar for LINAC-based and Gamma Knife systems. With both systems, it is possible to achieve dose distributions that conform closely to the shape of the intracranial target, thus sparing the maximum amount of normal brain. Recent advances in stereotactic imaging and computer technology for dose planning, as well as refinements in radiation delivery systems have led to improved efficacy, fewer complications, and a remarkable amount of interest in the various applications of SRS. Perhaps of equal importance is the fact that increasing amounts of scientific evidence have persuaded the majority of the international neurosurgical community that radiosurgery is a viable treatment option for selected patients suffering from a variety of challenging neurosurgical disorders including metastatic brain tumors (mets).

This chapter will present a brief description of radiosurgical technique. We will then present an evidence-based analysis of radiosurgery for mets.

Head Ring Application

Most stereotactic radiosurgical (single fraction) methodology requires attachment of a stereotactic head ring. The rigidly attached ring allows us to acquire spatially accurate information from computed tomography (CT) and magnetic resonance imaging (MRI). The images obtained with this ring establish fixed relationships between the ring and the target lesion that are later translated during treatment planning so that the treatment target is accurately placed at the precise isocenter of the radiation delivery device. Because the stereotactic head ring is bolted to the treatment delivery device, it also immobilizes the patient during treatment.

In general, patients are premedicated with 10 mg of oral diazepam given approximately one-half hour before ring application. Premedication is optional. No skin shaving or preparation is required. After the ring is assembled, with post drives and posts approximately positioned for application, the surgeon places the ring approximately in position. The post drives are moved in or out until the post tips rest loosely against the patient’s skin. As a rule, the front pin holes are positioned about an inch above the supraorbital ridges and in the midpupillary planes. The back pins are positioned just above the external occipital protuberance and approximately 2 inches from the midline. Having the patient slightly flex the head usually facilitates ring placement. In this position, the pins are usually perpendicular to the skull surface and therefore very unlikely to become dislodged.

As soon as the head ring is in final position for attachment, an assistant firmly stabilizes the ring from behind the patient while local anesthetic is injected through each of the post tip holes into the underlying skin. A wheal is raised with a solution containing equal parts of 0.5% lidocaine and 0.25% bupivacaine. This solution provides quick onset of anesthetic action as well as long duration.

Approximately 1 minute after anesthetic injection, the pins are inserted into the post holes and screwed through the skin until they rest against the skull. Using the pin wrench, we tighten the pins until the wrench cannot easily be turned using the thumb and first finger only. Care should be taken to avoid accidentally placing the pins into a burr hole, shunt, or onto a bone flap from a prior craniotomy. Occasionally, it is necessary to obtain skull fixation with three pins as opposed to the normal four because a large bone flap interferes.

At the conclusion of this procedure, the patient is transferred to a wheelchair and transported to the diagnostic radiology department for the next step (imaging) in the radiosurgery process.

Note well, that a number of papers describe radiosurgery using an external mask immobilizer instead of a head ring, along with infrared light–emitting diode or fluoroscopic tracking of head movement. Most of these treatments are multifraction, not single fraction.

Stereotactic Magnetic Resonance Imaging and Image Fusion

Stereotactic magnetic resonance images for use in radiosurgical treatment planning can be obtained in one of two ways: (1) by using a customized, MRI-compatible, head ring and localizer coupled to a specially tuned MRI coil to minimize the spatial inaccuracies that result from perturbation of the magnetic field, or (2) through the use of computer-generated image annealing software programs, commonly termed image fusion .

Image fusion techniques allow MRI images acquired without the stereotactic head ring to be used for treatment planning. The MRI scan used for image fusion is routinely obtained the day before treatment. Images acquired for image fusion use the standard diagnostic MRI head coil, and the scan is not limited to the area of interest but includes the entire head. The scan technique uses volumetric image acquisition with a modified T1-weighted sequence. This technique allows rapid image acquisition so that movement during the MRI is minimized. Image fusion eliminates many of the hardware incompatibility problems involved with using MRI for treatment planning. The volumetric scan technique also allows sub-millimetric slices, similar to the CT technique. Image resolution is identical to that used for diagnostic MRI scanning. Fig. 88.3 illustrates an example of an image-fusion treatment database being compared with a CT treatment database before beginning treatment planning.

Radiosurgery Treatment Planning

Once the necessary stereotactic images have been acquired and transferred to the treatment-planning computer, the next step is to plan the precise delivery of radiation. This is accomplished through the use of a computer workstation and specialized treatment planning software “tools.”

Goals of Radiosurgery Treatment Planning

An ideal radiation treatment plan would deliver 100% of the desired dose to the treatment target and none to the normal brain. This is not possible in reality, but the primary goal of radiosurgery treatment planning is to achieve a plan that conforms to the target as closely as possible, as defined by radiation isodose shells . Isodose shells are volumes bounded by surfaces that receive the same radiation dose—expressed as a specified percentage of the maximum radiation dose. A number of treatment planning tools are available for adjusting the shape of treatment isodose shells so that they fit even highly irregular target shapes. Regardless of its shape, the entire target must be treated within the prescription isodose shell (most commonly the 70% or 80% line), with as little normal brain included as possible ( Fig. 88.4 ).

Another goal of dose planning is to adjust the dose gradient such that critical brain structures near the target receive the lowest possible dose of radiation.