Brain Surgery Information:

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CHAPTER 14: THE RADIOSURGICAL PROCEDURE AND EARLY POSTOPERATIVE PERIOD

Stereotactic radiosurgery (SRS) involves the delivery of focused beams of radiation to a target such as a brain tumor. Although the word surgery is used, there is actually no skin incision, no open operation. SRS is really radiation therapy administered by a neurosurgeon or radiation oncologist with surgical precision. SRS has been used now for a few decades and in a few hundred-thousand patients. It is generally regarded as being safe and effective for a wide variety of brain conditions, including certain tumors and some AVMs. However, it is not without complications (Chapter 18). The reasons that SRS has not replaced open surgery as the gold standard for the majority of brain lesions include that SRS: (1) Typically does not remove lesions such as tumors, but rather tries to stop or arrest their growth by killing off their rapidly dividing cells; (2) is not as useful in the setting of large lesions, especially those greater than 3 cm in diameter, or those that are large and cystic or debris-filled; (3) does not alleviate the bad effects of lesions exerting significant “mass” within the brain, that is, lesions causing brain shift or impending herniation (Chapter 5); (4) is not effective for all lesions, as some are naturally radiation-resistant; and (5) can take months or a few years to work its effects on radiation-sensitive tumors or AVMs, respectively, and this timeframe may not be suitable for certain patients owing to the risks associated with such conditions during the time in which the patient is awaiting possible “cure”. Having stated all of this, however, it cannot be denied that the availability of SRS has certainly played a very beneficial role in the treatment of many patients, especially those with: (1) Brain tumors, both primary and metastatic, especially after surgical “debulking”; and (2) lesions that are very deep in the brain, or spreading along the skull base.

There are several different SRS systems available to patients. Two very popular SRS systems discussed below are the Leksell Gammaknife® and the Accuray Cyberknife®. These systems are used in an outpatient setting, that is, half-day procedures with no hospital admission required. Although the systems can deliver all the radiation in one “sitting”, sometimes the physician elects to have the lesion treated over a few sittings, something referred to as hypofractionation (HF). This may be safer for certain lesions near very critical structures, and HF may also be used for slightly larger lesions.

·  Gammaknife® (GK): For the GK system, the patient is brought to the SRS suite, and local anesthetic is injected into 4 sites on the scalp, two at the front of the head and two at the back. Frequently, an oral or IV relaxing medication is administered, along with a dose of anti-inflammatory steroid. A relatively light-weight metallic head frame in the shape of a hollowed out cube is placed over the patient’s head and the pins are advanced by wrench-tightening into the numbed scalp (Figure 25). If there is any pain during pin tightening, more local anesthetic is administered by the neurosurgeon. There may be some tight pressure as the pins are advanced to the skull’s outer surface, but patients report that they adapt to this within minutes, and for the remainder of the procedure are comfortable. There is no need for intubation. After placement of the headframe, the patient is taken to the MRI or CT scan unit, and the scan is taken with the head frame incorporated into the scanned images. The images are verified by the neurosurgeon, and the patient returns awake to the GK suite. At this time, other members of the SRS team, including a radiation oncologist and a medical physicist, along with the neurosurgeon, use a sophisticated software program to plan the dose and configuration of the radiation to be administered. The brain images and head frame coordinates obtained from the head scan are used to create the safest and most precise or conformal dose plan. Once the plan is verified, the patient is placed on the GK table, which is like that of a CT or MRI scanner, and a hair-salon style helmet is placed over the headframe. This “collimated” helmet is metallic and has over 200 openings in it through which the fine radiation beams will pass. The patient, with helmet on, is advanced on the table into the doughnut-shaped radiation gantry of the GK unit, awake and freely able to talk with the GK staff, and at the same time be monitored by them throughout the procedure. The radiation “shots” are administered in short stages, and the patient’s head position frequently needs to be changed. The radiation delivery itself typically takes a few hours to complete. At the end of the procedure, the patient is observed briefly and then dismissed to home. It is recommended that the patient have someone accompany them in order to safely transport the patient home at the end of the day. Appropriate followup should be discussed and/or arranged by the surgeon before the patient leaves the unit.

·  Cyberknife® (CK): For the CK system, there are no pins, no head frame, and no helmet. There is no need for any anesthesia or “premedication” except for a dose of steroid per the physician’s preference. The CK suite includes a radiation device called a mini-linear accelerator or Linac to deliver the radiation, and this is mounted on a computer-controlled robotic arm (Figure 26). Uniquely, the system uses real-time image guidance technology to monitor the patient’s breathing movements and any head or torso movements, and can adjust or fine-tune the Linac’s direction accordingly, in order to maintain its safety and accuracy. Instead of an invasive head frame, the CK system uses a flexible breathe-through mesh face mask and/or torso frame to keep the patient as still as possible during the procedure. The CK system also has the advantage of being able to be used throughout the body, that is, for tumors and other lesions in the spine, chest, abdomen, pelvis, and so forth. Once the radiation plan is made with the help of the CT or MRI scan, the radiation delivery takes place over a period of 60-90 minutes. No alteration of the patient’s position is required to carry out the delivery. The patient is observed briefly, and then dismissed to home. Again, followup should have been arranged.

Complications of SRS are discussed elsewhere (Chapter 18). Interestingly, advocates of the GK system claim that the headframe makes the targeting more accurate compared with the CK system. On the other hand, advocates of the CK system claim that it is as accurate as the GK system, avoids placement of a headframe and helmet, and can be used for different body regions. Overall, many neurosurgeons feel that these two excellent systems are very comparable for the purpose of focused brain radiation delivery.

Figure 25 (above). Gammaknife® head frame for accurate targeting.

Figure 26 (above). Cyberknife® robotic arm for accurate Linac radiation targeting. Illustration adapted from www.accuray.com.