X-Knife Radiosurgery Advances at the Cedars-Sinai Comprehensive Cancer Center

Stereotactic radiosurgery (SRS) is a focused radiation technique used to precisely treat small benign or malignant tumors of the brain or skull base. "Stereotactic" refers to the accurate three-dimensional (3-D) localization of a given target. Reproducible immobilization of the patientās head in a fixed frame or "halo" allows for the safe delivery of a series of overlapping radiation arcs. These beams act as a scalpel to inactivate a carefully circumscribed area of abnormal tissue, while sparing adjacent normal brain tissue.

X-Knife 3 is the Harvard-developed software system we use to plan our radiosurgery cases. Another commonly used apparatus is Gamma Knife. This is an older, single-function system that uses a helmet with numerous radioactive pieces of cobalt that are directed toward its center. X-Knife instead uses a multi-purpose linear accelerator and noncoplanar beam arcs as described above. As a result, treatments are less costly, and a broader range of target sizes can be treated with the X-Knife System.

The situations in which SRS is beneficial are still being studied. The maximal tumor diameter we treat is 4 or 5 cm (about 1.5 -2 inches), for single or fractionated treatments, respectively. Single fraction radiosurgery (SRS) is delivered in one day, with a headframe fixed to the skull by a neurosurgeon prior to imaging and treatment. This is performed with local anesthesia, and removed the same day. Fractionated "stereotactic radiotherapy" (SRT), by contrast, is delivered in multiple sessions with a removable headframe.

This relocatable SRT headframe, called the Gill-Thomas-Cosman (GTC), uses a dental mold, a mold for the back of the head and velcro straps to create millimeter reproducibility without requiring pins or anesthesia. It is especially useful in treating malignant tumors, where normal tissue can repair between treatments. Tumor tissue, with its abnormal genetic material, is less able to repair such damage. Thus "fractionated" SRT with the GTC headframe further minimizes toxicity, and is essential in treating larger targets or those close to critical radiosensitive structures (e.gs., the brainstem, optic nerves and chiasm). Such tumors present too great a risk for single-fraction SRS.

Tumors commonly called "benign" that are treated by SRS include acoustic neuromas, arteriovenous malformations (AVMs), craniopharyngiomas, pituitary adenomas, and meningiomas. Malignant targets include skull base carcinomas, high grade gliomas and metastases. Metastases are tumors that spread to the brain via the bloodstream from primary sites elsewhere in the body.

Many of the diagnoses listed are treated in conjunction with neurosurgery, chemotherapy or conventional radiation. A multidisciplinary team at Cedars-Sinai evaluates potential patients and recommends the optimal combination of treatments.

Patients are selected for SRS or SRT according to a variety of criteria in addition to diagnosis. These include their functional status, age, life expectancy and the size and location of the tumor(s). Those with more than four brain metastases are considered unlikely to benefit, and are thus treated solely by conventional brain irradiation.

SRS patients typically arrive in our department in the morning on the day of treatment. Headframe placement is followed by a CT scan (and then an angiogram if the diagnosis is AVM). Patients wait in a private room in our Treatment Area while a team of specialists, consisting of a radiation oncologist, neurosurgeon, neuro-radiologist and radiation physicists, plans the treatment and performs painstaking quality assurance. Treatment is then delivered, typically taking between 45 and 90 minutes. The headframe is then removed, and the patient discharged home. The entire procedure is conducted in an outpatient setting, minimizing costs and inconvenience.

SRT planning and treatments are spread over time. The first day, patients undergo headframe placement followed by CT scanning. They then go home while planning and quality assurance measures are conducted. Each time they return, the GTC frame is again placed, and a treatment delivered. Typical fractionation schemes involve five or six treatments over three weeks, but vary according to diagnosis, size, location and prior treatment history.

Risks from radiosurgery are largely related to edema (swelling) in the target area. This can be an acute exacerbation of tumor-related inflammation, or late radiation necrosis. Necrosis (tissue death) can affect normal tissue directly, when included in the radiosurgery target, or more commonly represents death of tumor tissue. Regardless of its source, brain edema can result in a variety of neurologic symptoms. These are unusual, but are most often temporary and controlled by a course of steroid medication. Rarely, patients require surgical removal of such edematous tissue. In a study of high grade gliomas at UCSF, implants patients who required such removal actually outperformed those who did not, implying that the potentially toxic dose that they received had proved beneficial.

Another exciting advance seen in X-Knife 3 is "fusion capability". This allows physicians to join CT and MRI (magnetic resonance imaging) data for treatment planning. Any recent MRI available on digital tape can thus be used in conjunction with the planning CT conducted with the headframe in place. CT data is useful because there is minimal geographic distortion, and MRI often enhances resolution of the target and critical structures. Fusion software thereby maximizes patient convenience and further minimizes costs by obviating the need for unnecessary scan duplication.

In summary, radiosurgery offers an alternative to more invasive procedures in carefully selected patients with small benign or malignant tumors of the brain and skull base. Patients whose tumors were previously considered untreatable, due to their location deep in the brain or the patientās poor medical condition, can now be offered this remarkable therapy. And the multidisciplinary team at the CSCCC, with its cutting-edge X-Knife 3 technology, ensures optimal accuracy and flexibility in patient evaluation, treatment and follow-up. For additional information, have your physician call our clinical coordinators at (310) 855-4206.