Stereotactic Radiosurgery

In 1951, Dr Lars Leksell coined the term "stereotactic radiosurgery" (SRS), which is the delivery of a single, high dose of radiation via stereotac-tically directed beams into a small target. Presently, three main types of SRS techniques are used: heavy charged particles (proton beam), gamma irradiation from cobalt-60 sources (gamma knife), and high-energy photons from linear accelerators (LINACs). With all of these methods, a stereotactic head-frame is used for patient positioning and target determination.

In the past 10 years, SRS has become an increasingly popular option in managing meningioma patients with tumors that are less than 3 cm in diameter. This technique is more advantageous than surgery for a number of reasons. It is minimally invasive, does not require general anesthesia, and can be performed as an outpatient procedure. In addition, it carries a minimal risk of bleeding and infection, and recovery time is minimal. Stereotactic radiosurgery also costs less than conventional surgery.

The gamma knife (Elekta Instruments, Atlanta, GA) is a dedicated machine that performs SRS. The machine uses 201 cobalt-60 sources arranged in a hemispheric dome around the patient's head. Since 1968, more than 100,000 patients with various tumors, vascular malformations and functional disorders have been treated with this device. The goal is to encompass the target area using multiple isocenters or shots to maximize conformality and to minimize the radiation dose to normal surrounding structures. The initial results from gamma knife radiosurgery have been encouraging. Kondziolka et al. reviewed the long-term results of 99 consecutive patients (89% had skull base tumors) who underwent gamma knife radiosurgery for meningiomas between 1987 and 1992 [32]. The patients were assessed using follow-up scans, patient survey, and physician-based evaluations. Using an average tumor dose of 16 Gy (range 9-25 Gy), the authors achieved clinical tumor control in 92 of the patients (93%). Sixty-one (63%) of the tumors decreased in size. Two factors seemed to predict local tumor progression: history of prior resection (P = 0.02) and history of multiple meningiomas (P<0.00001). The mean actuarial rate of post-radiosurgery complications was 4.85% at 31-120 months. Overall, 96% of the surveyed patients were satisfied with their outcome. Morita et al. [33] reviewed the results of a prospective study of 88 skull base menin-giomas treated by gamma knife radiosurgery. With a median follow-up time of 35 months, the progression-free survival rate was 95% [33]. Follow-up scans showed that 68% of the tumors had decreased in size.

Modified and dedicated LINAC-based systems are also commonly used for SRS. To achieve multiple, convergent beams of radiation, the couch and gantry of the LINAC are rotated around the isocenter or target. This achieves the sharp, dose-gradient characteristic of SRS. The results of LINAC radiosurgery seem comparable to those of gamma knife SRS. Shafron [34] reported on 70 patients with 76 meningiomas. After a median follow-up of 23

months, no lesion had enlarged, and 21 out of 48 (44%) had decreased in size after at least 1 year of follow-up. Hakim et al. [35] reported on 106 benign meningiomas that were treated with LINAC radiosurgery. The 5-year actuarial tumor control rate was 89.3%.

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