Research and Outcomes in Neurosurgical Radiosurgery
Excerpts from “Brain Talk, Volume 6, Number 1”, International Radiosurgery Support Association, Harrisburg, PA
Arteriovenous Malformations
“Stereotactic radiosurgery has had a wide application in the management of cerebral arteriovenous malformations (AVMs). This technique has been used in patients with small to medium sized AVMs in all brain locations, but perhaps its greatest role is in the management of AVMs in critical brain locations. The roles of microsurgical resection, embolization and radiosurgery are now defined more clearly so that physicians and patients can choose appropriate strategies more specifically. The goals of radiosurgery is complete obliteration of the AVM nidus achieved through proliferation of endothelial cells lining the vessel wall, its progressive thickening and eventual blood vessel closure. It usually takes one to three years before obliteration occurs.
“We have managed over 810 patients with arteriovenous malformations of the brain. An additional 84 patients had radiosurgery for cavernous malformations. All patients selected for radiosurgery had a well-defined vascular malformation as seen on preoperative high-resolution angiography or MRI scan. Patients ranged in age from 2 to 82 years old; the average age was 36.
“Successful AVM obliteration is dependent upon good target definition, the radiosurgical dose administered and the AVM’s volume. Problems with target definition are the single greatest reason for failure to achieve obliteration. Multiple, three-dimensional imaging including MRI, MRA, and angiography is crucial to obtain the best result. For the smallest AVMs (less than 1.3 ml), 90% of patients had complete obliteration and 98% had obliteration of the target. The obliteration rate lessens for larger AVMs but remains above 70%. Again, the radiosurgical dose must be appropriately tailored to the AVM margin as defined by multimodality imaging.
“Immediate post-radiosurgery complications are rare. Six percent of patients had a new neurologic deficit after radiosurgery that in most cases was temporary.
“Staged procedures (usually two to three procedures spaced by 3 to 6 months) for selected large AVMs is a new therapy for surgically untreatable AVMs. Such a program is under evaluation at our center, and 17 patients have now been treated.”
Acoustic Neuromas
“We have managed 651 patients with acoustic neuromas as an alternative to microsurgical resection according to one or more of the following criteria: advanced patient age, poor medical condition for surgery, recurrent or residual tumor after prior surgery, neurofibromatosis type II (a disorder involving swellings that grow from nerves) or patient preference.
“Our review of patients with acoustic tumors with 5 to 10 years follow-up (162 patients managed between 1987 and 1992) was compared to our published review of 190 patients managed between 1992 and 1997. We found that the clinical tumor control rate (no requirement for surgical intervention) for the first group was 98%, and 97% for the second.
“Since 1987, there have been significant modifications in the technique of acoustic tumor radiosurgery including a change from CT to MRI based planning, improved computer workstations, the use of more isocenters of radiation, the use of smaller irradiation beams, and a reduction in the radiation dose. Since the institution of these techniques beginning in 1991, there has been a significant reduction in the morbidity of radiosurgery. Since 1992, the risk for a delayed, mild facial weakness is below 1% and for facial numbness, below 3%. A survey of these patients with at least five-year follow-up found that 97% believed radiosurgery was a good treatment for their acoustic neuromas.
“Radiosurgery has been shown to be a cost-effective alternative to microsurgery for acoustic neuroma patients in a recent outcome analysis.”
Meningiomas
“Stereotactic radiosurgery is applicable for patients with intracranial meningiomas because these tumors are usually well defined and rarely invade the brain unless malignant. The steep radiation fall-off can be directly targeted to the tumor margin. Meningiomas are well visualized on imaging studies and can be recognized even when small. As many tumors exist at the skull base where a total resection is often not feasible, radiosurgery can be used as a second stage approach after prior microsurgical resection to treat residual tumor in critical brain locations such as the cavernous sinus. Delayed tumor recurrence after surgery, surgical morbidity and surgical mortality, especially in the elderly, increasingly has led to consideration of radiosurgery for primary or adjuvant treatment of selected meningiomas.
“We used radiosurgery in 584 patients with meningiomas. Long-term prevention of tumor growth remains above 90% for these patients. Five percent have had imaging-defined tumor enlargement after radiosurgery. Two patients treated before 1990 had delayed visual symptoms after radiosurgery: one had a temporary decrease in visual acuity that returned to normal and another a delayed persistent hemianopia (visual field loss). Since a rigid protocol restricting the dose to the optic apparatus to less than 8 Gy was begun 10 years ago, no meningioma patient has had a delayed visual deficit. Many believe that aggressive skull base surgery is no longer required for patients with small or medium sized tumors and that radiosurgery alone or resection plus radiosurgery (for larger tumors) is a better choice.”
Brain Metastases
“Radiosurgery perhaps has had its greatest recent role in the management of patients with metastatic brain tumors. Local control provided by radiosurgery for the management of tumors in any brain location exceeds 85% on average. Patients with four or fewer tumors are usually good candidates for radiosurgery. As a minimally invasive procedure that provides excellent local control, relief of symptoms, and avoids open craniotomy, radiosurgery is an attractive and cost-effective treatment for both patients and physicians managing these problems.
“To date, the results after radiosurgery plus whole-brain radiation therapy appear equal to surgical resection plus whole-brain radiation. Radiosurgery is effective for tumors traditionally considered ‘radiation-resistant,’ including melanoma and renal cell carcinoma. To date, we have managed 788 patients with brain metastases.
“Radiosurgery has been shown to be a cost-effective management strategy compared to either surgical resection or whole brain radiotherapy for patients with solitary brain metastases.”
Malignant Gliomas
“Radiosurgery has been used to provide a radiation boost in the management of patients with malignant glial tumors, in addition to a regimen of fractionated radiation therapy. Increasing interest in interstitial irradiation (brachytherapy) through the 1980’s was followed by interest in the use of radiosurgery as an alternative method of providing a local radiation boost without the need for invasive catheter placement. At our center, most patients with malignant gliomas first undergo stereotactic biopsy or image-guided resection. Initial adjuvant therapy consists of fractionated radiation therapy and sometimes chemotherapy. Radiosurgery is used as an additional treatment for patients with contrast-enhancing masses less than 3.5 cm in average diameter.
“We believe that radiosurgery plays an important role in the multi-modality management of patients with malignant glial tumors. In our experience, median survival after radiosurgery (not diagnosis) was 16 months for patients with glioblastoma and 21 months for patients with anaplastic astrocytoma.
“When compared to standard radiation therapy survivals reported by the Radiation Therapy Oncology Group, significant benefits were obtained after radiosurgery.”
Pituitary Tumors
“An evaluation of our series of 155 patients who had stereotactic radiosurgery for pituitary tumors was performed. Two-thirds had hormone-secreting tumors. Tumor growth control was achieved in 94% of patients and 46% noted a decrease in tumor size. Most of the time, radiosurgery is performed following a partial transphenoidal resection, or after a craniotomy. Radiosurgery should be considered a first choice if the tumor clearly involves the cavernous sinus region.
“For patients with acromegaly (growth hormone secreting tumor), normalization of growth hormone levels was identified in 72% of patients and a significant reduction was identified in an additional 21%. For patients with Cushing’s disease (adrenocorticotropic hormone secreting tumor), normalization of pituitary function was identified in 52% and another 14% showed a significant decrease in section.
“New neurologic problems after radiosurgery were rare. Two patients in our early experience had visual deficits: one patient who had multiple prior surgeries and radiation therapy experienced hypothalamic deficit, and the other had new onset facial sensory loss. No patient developed additional pituitary hormone deficits after radiosurgery.”
Other Benign Tumors
“Radiosurgery has been used to manage patients with an array of other benign tumors that include craniopharyngiomas, hemangioblastomas, trigeminal nerve and jugular foramen schwannomas, choroids plexus papillomas, and neurocytomas. Tumor volume and location are important factors for the selection of radiosurgery over other therapeutic alternatives. Although these tumors are less common, there is an increasing experience in the use of radiosurgery, usually after a biopsy or resection has confirmed the diagnosis. The response rate to radiosurgery appears high, as for other benign tumors.”
Other Malignant Tumors
“Malignant tumors of the skull base can be treated effectively with radiosurgery. Such tumors include chordomas, chondrosarcomas, nasopharyngeal carcinomas, or other adenocarcinomas or squamous cell carcinomas from regional structures. We use radiosurgery for residual or recurrent cancers that invade the cranial base. Because the radiation dose can be delivered in a single session and spare surrounding critical brain structures, radiosurgery has potential advantages over fractionated techniques or radiation implants for the management of malignant skull base tumors. To date, radiosurgery is often selected after other procedures or treatments have failed. Complications from treatment are unusual. Other malignant tumors of the brain include germinomas and non-germinomatous germ cell tumors, and CNS lymphomas. Germinoma, a radiosensitive tumor, appears to respond quickly and dramatically to radiosurgery.
“Further evaluation will be necessary in a larger series of patients to define the response of different tumor histologies in order to more clearly determine the role of radiosurgery in contrast to other surgical techniques or radiation delivery methods.”
Trigeminal Neuralgia
“On initial use of stereotactic radiosurgery was for lesion generation in functional neurosurgical procedures. Increasingly, Gamma Knife radiosurgery is used to treat trigeminal neuralgia (TN). We have treated 383 TN patients to date. Radiosurgery was performed sparingly for this indication until recently, when high resolution imaging of the trigeminal nerve provided excellent definition of the target. At present, we use a maximum radiosurgical dose of 80 Gy for most patients. This provides a significant rate of pain relief, with an approximate 10% chance of mild facial numbness.
“Initial experience has mainly been in patients with recurrent pain after prior surgery. Approximately two-thirds of patients became pain-free of medication and another 20% obtained significant benefit. TN radiosurgery requires the use of small collimators (4 mm) and highly precise targeting. At this time, only Gamma Knife techniques have been validated for TN radiosurgery.”
Movement Disorders and Tremor
“Since the Gamma Knife can be used to create a focal radiosurgery brain lesion, it can be used to perform thalamotomy, a procedure used to treat tremor. We consider this for patients who have medical contraindications to open thalamotomy or to deep brain stimulation. We have performed 44 radiosurgical thalamotomies for tremor (Parkinson’s disease, essential tremor, and multiple sclerosis tremor). The majority of patients note significant tremor relief 1 to 3 months following radiosurgery. Rarely, hand numbness or weakness can develop in the months after the procedure. Radiosurgical pallidotomy, used to treat other symptoms of Parkinson’s disease, has been performed at some centers, but we favor conventional radiofrequency pallidotomy.”
