Stereotactic Radiosurgery
Surgeons use energy in many forms to cure disease. Scalpels, lasers, and electrocautery were the initial tools of the neurosurgeon. Recent advances in neuroimaging, computer science, and stereotactic dose planning allow neurosurgeons to use sculpted radiation fields to alter the biology of disease. Stereotactic radiosurgery is the mechanically precise delivery of a potentially cytostatic, obliterative, or functionally incapacitating single dose of radiation to an imaging-defined target volume while minimizing risk to surrounding tissues. The goal of radiosurgery is to alter the molecular physiology to effect a positive change on the disease process. Radiation transfers energy to its target, initiating a cascade of high-energy electrons that interact with matter ultimately to arrest tumor growth, alter the blood supply of tumors or vascular malformations, or ablate undesirable nerve conduction. The unique design of stereotactic radiosurgery systems allows small areas of the brain to be treated with high doses of radiation. The sharp fall-off of radiation dose prevents brain outside of the target area from receiving deleterious doses.
Radiosurgery is a ”patient friendly” procedure that does not require open surgery and allows the patient to be discharged the day after surgery. This does not imply that radiosurgery is noninvasive or risk free. Successful radiosurgery requires a multidisciplinary team, including a neurosurgeon, a radiation oncologist, a medical physicist, a nurse, and an administrator. We review the current indications for Gamma Knife radiosurgery, treatment strategies, results, and complications for common neurosurgical indications.
The Gamma Knife design is a unique tool for the neurosurgeon. Two hundred and one sources of cobalt 60 radiation are directed to a focal point. The Leksell stereotactic frame allows patients to be positioned within the unit with 0.5-mm accuracy. Helmets with 201 collimators of 4, 8, 14, or 18 mm allow conformal shaping of the radiation field. The design is excellent for treating targets within the skull, down to the foramen magnum. The number of patients treated with the Gamma Knife has grown exponentially since its innovation in 1967. Currently, there are 51 Gamma Knife units in the United States, and 126 units worldwide. As of June 1999, more than 100,000 patients have been treated (Table 1).
Frame Application and Imaging
The frame is applied in approximately 5 minutes in the stereotactic suite using local anesthesia, occasionally supplemented by intravenous sedation (midazolam and fentanyl). Children younger than age 12 generally undergo general anesthesia for stereotactic procedures. Patients are positioned supine on the stretcher with the head of the bed elevated 60°. The chest is supported with several pillows to flex the body 90 degrees at the waist, allowing greater access to the head. Ideally, two persons assist with frame placement. A nurse helps to stabilize the head while the surgeon and assistant attach the frame. After the head has been prepared with isopropyl alcohol and the patient is comfortably sedated, ear bars are placed into the external auditory canal with a 1-cm square foam pad on the end of the ear bar. The foam padding alleviates patient discomfort during frame application. The ear bars are intended to assist with symmetrical alignment and do not bear the weight of the frame. The surgeon supports the frame throughout the application.
The frame is shifted toward the side of the lesion to position the lesion as close as possible to the center of the frame and the collimator helmet. Lidocaine (0.5%) and bupivicaine (0.5%) buffered with sodium bicarbonate are injected into each pin site. The pin length is chosen to attach the frame bars with finger-tight torque and without pin protrusion beyond the frame. During imaging, a rigid frame adapter keeps the frame orthogonal to the imaging plane.
Table 1 Mean Marginal Doses and Results
|
Lesion |
Marginal dose range |
Maximum dose |
Results |
|
AVMs (<4 cm3) |
18-25 Gy [10] |
|
88% obliteration at 3 years |
|
Acoustic neuromas |
12-14 Gy |
97% tumor control at 10 years |
|
|
Meningiomas |
16 Gy |
93% tumor control at 5-10 years |
|
|
Metastasis |
10-27 Gy |
86% local tumor control |
|
|
Pituitary tumors |
10-30 Gy |
60%-90% tumor control |
|
|
Trigeminal neuralgia* |
80 Gy |
80% pain relief |
|
|
Tremor* |
120-140 Gy |
80% reduced tremor |
*The dose used for trigeminal neuralgia and tremor represents the maximal dose within a single 4-mm isocenter. AVM, Arteriovenous malfunction.
A sagittal localizing film is initially obtained. A contrast enhanced volume acquisition magnetic resonance imaging (MRI) scan is obtained with 1-mm slice thickness images, or 3-mm images for selected lesions. In imaging arteriovenous malformations (AVMs), contrast is given after the localizing sagittal image to assure a maximal contrast load during the axial image acquisition.
DOSE PLANNING
Gamma Knife dose planning uses one or more (sometimes many) small isocenters of radiation to create a conformal plan that encompasses the lesion and minimizes the dose received by surrounding structures. The Gamma Knife provides 4-, 8-, 14-, and 18-mm collimators. Similar conformal plans can be designed from one or more large isocenters or from the combination of a greater number of smaller isocenters. The plan created with the larger isocenters will deliver an equivalent dose of radiation, but in a shortened time because of the larger aperture of the collimator. Additionally, the radiation falloff into the surrounding tissues may not be as steep with larger collimators as compared with the smaller collimators. Conformal dose planning is particularly important when working near critical structures, such as the brainstem or optic nerves. Conformal radiosurgery is achieved through judicious selection of appropriate isocenters, proper interpretation of imaging, and dose selection.
The Gamma Knife provides the option of beam blocking as an additional tool for sculpting the fall-off pattern to protect surrounding structures, or to eliminate the passage of beams through important structures such as the ocular lens. Each of the 201 collimator ports on the helmet can be blocked. Blocking a group of beams requires that a greater dose of radiation be passed through the remaining beams to achieve the same target dose. As a consequence, the fall-off is shifted along the axis of the remaining beams. For example, blocking the entrance of beams entering from the left and right, the fall-off is diminished in the left-right axis, and shifted to the rostro-caudal, or antero-posterior axis. A limitless variety of blocking combinations is possible to achieve the best result. The effect of various blocking patterns is quickly evaluated on the planning software, before implementing the plan. We most often use blocking patterns to protect the ocular lens optic apparatus.
ARTERIOVENOUS MALFORMATIONS
The best AVM for radiosurgery is not necessarily the same AVM that is treated by conventional surgery. The ideal candidate for radiosurgery is a patient with a small, deep-seated AVM, not a patient with a larger AVM who was considered unsuitable for microsurgery. In evaluating AVMs for radiosurgery, the Spetzler-Martin scale may not apply, as it is not sensitive to smaller volumes. The success of radiosurgery is not as limited as microsurgery by critical locations or the venous drainage. The major shortcoming of radiosurgery for AVMs is the persistent risk of hemorrhage until the AVM is obliterated. Before treatment, the risk of hemorrhage is estimated to be 2% to 4% per year. The risk of hemorrhage is not increased by radiosurgery.
Proper imaging of the vascular malformation is required for successful radiosurgery. Imaging of AVMs includes a volume acquisition MRI as well as conventional angiography. Both modalities are needed to distinguish the nidus from draining veins and nearby critical structures. Embolization has been evaluated in conjunction with radiosurgery and shown to have a 12.8% morbidity, 1.6% mortality, and 11.8% 1-year recanalization rate [1]. Given the morbidity and sometimes limited efficacy, we do not routinely advocate embolization before radiosurgery.
The radiosurgical dose given to AVMs is a function of the volume, location, and risk assessment. Giving a higher dose increases the probability of AVM obliteration, but may increase the risk of side effects. University investigators reviewed the histology of AVMs that were resected after ra- diosurgery and found that a dose of more than 20 Gy was necessary for the desired histological effects (endothelial injury, hyperplasia, and thrombosis). After 4 years, there was not much additional effect. In our own experience, AVMs that have not occluded within 3 years of radiosurgery require additional treatment. The probability of AVM obliteration is approximately 98% for volumes treated with a minimum dose of 25 Gy, and 90% with a minimum dose of 20 Gy. Arteriovenous malformations that are larger than 10 cm to 15 cm3 may require irradiation in staged volumes with a 3- to 6-month interval between procedures (Fig. 1).
ACOUSTIC NEUROMAS
The goals of acoustic neuroma management include tumor control (radio-surgery) or complete removal (microsurgery), preservation of facial nerve function, and maintenance of ”useful” hearing in appropriate patients. A comparison of microsurgery and radiosurgery from 1990 to 1992 revealed that hearing was preserved in 14% of microsurgery procedures and 75% of Gamma Knife procedures [2]. Normal facial function was achieved in 63% of microsurgical procedures and 83% of Gamma Knife procedures. Since then, radiosurgery results have improved significantly.
Radiosurgery is performed with an axial volume acquisition contrast-enhanced MRI. The treatment of acoustic neuromas at our institution has evolved over the past 10 years. Initially, the tumor margin was treated with approximately 18 Gy to 20 Gy, with the center of the tumor receiving up to 40 Gy. The marginal dose has been gradually reduced to 12 Gy to 14 Gy, allowing a significant reduction in complications and continued tumor control. We usually treat the tumor margin with 50% of the maximal dose. For patients between 1987 and 1992, the 10-year tumor control rate was 98% with normal facial function in 79% and normal facial sensation in 73% of patients [3]. For patients managed between 1992 and 1997, the control rate was 99%, and the facial nerve morbidity was 1%. More than 60% of patients had a reduction in the volume of their tumor. We have treated 10 patients with intracanalicular acoustic neuromas with 14 Gy or less to the margin and preserved hearing for all patients (Fig. 2) [4].
MENINGIOMAS
Meningiomas of the falx, convexity, olfactory groove, and lateral spenoid wing can be treated with radiosurgery, but they can also be resected with low morbidity. Meningiomas that are not as easily resected, such as those along the petrous apex and clivus or extending into cavernous sinus, are often optimal radiosurgical cases. The steep fall-off of radiosurgical dose planning allows these difficult tumors to be treated with marginal doses of 13 Gy to 16 Gy, with a low risk of injury to the associated cranial nerves, pituitary gland, or optic nerve. As for other indications, we generally prescribe a marginal tumor dose that is 50% of the maximal dose.
Figure 1 A 45-year-old man with a right temporal arteriovenous malformation declined operative resection. The radiosurgery plan demonstrates the use of both magnetic resonance imaging and angiography for dose planning. The combined imaging allows more precise planning and exclusion of the large anterior superior draining vein. A maximal dose of 36 Gy and marginal dose of 18 Gy were used.
Using an average marginal dose of 16 Gy, we have achieved long-term tumor control in 93% of meningiomas, more than half of which had been previously resected. Neurological deficits occurred in only 5%. Overall, 96% of patients surveyed believed that radiosurgery provided a satisfactory outcome for their meningioma [5]. We believe that planned judicious microsurgery, when indicated, followed by radiosurgery may improve overall clinical outcomes (Fig. 3).
Figure 2 This 64-year-old woman had a gross total resection of a right acoustic neuroma 8 years earlier. The recurrent tumor was treated a maximal dose of 26 Gy, with a marginal dose of 13 Gy. The conformal plan included seven 4-mm isocenters.
PITUITARY TUMORS
Microsurgery remains the procedure of choice for the rapid treatment of pituitary tumors that are causing mass effect or secreting adrenocorticotropic hormone or growth hormone. Stereotactic radiosurgery is an effective alternative for patients who do not require decompressive surgery, rapid normalization of endocrine abnormalities, or who suffer from recurrent tumors despite medical and surgical intervention. The treatment goals for pituitary tumors are control of tumor growth, cessation of abnormal hormone secretion, and avoidance of neurological injury. We have achieved tumor control in 94% of tumors [6]. In our series, cortisol secretion was normalized or reduced in 62.5% of patients. Growth hormone levels were normalized in 67% of patients and significantly improved in most of the remaining patients. None of our patients developed pituitary insufficiency as a consequence of radiosurgery.
Figure 3 A petroclival meningioma in a 61-year-old woman who had diminished hearing and dysequilibrium. The conformal plan with a maximal dose of 28 Gy and marginal dose of 14 Gy used one 14-mm iso-center, six 8-mm isocenters, and one 4-mm isocenter.
The greatest risk in treating pituitary lesions with radiosurgery is damage to the optic nerves and optic chiasm. We recommend radiosurgery only for those tumors that are at least 1 mm to 2 mm away from the optic nerve. The steep fall-off of radiosurgery units allows pituitary lesions to be treated with 40 Gy to 50 Gy maximal doses (20-25 Gy marginal doses) while exposing the optic nerve to less than 8 Gy. Lesions in the sellar area provide an excellent demonstration of the unique radiation fall-off pattern for the different Gamma Knife units. The "B" unit has a steeper fall-off in the rostral-caudal dimension, whereas the earlier "U" or "A" unit has a steeper fall-off in the left-right dimension. This differential can be exploited, depending on the configuration of the tumor and the surrounding structures. Additionally, some of the collimators on the helmets can be blocked to alter the fall-off pattern.
