Treatable Conditions

There are two types of brain tumors. Primary brain tumors arise from the tissues which make up the brain and its coverings. Primary brain tumors may be malignant or benign. Secondary, or metastatic brain tumors, travel to the brain from cancerous tumors elsewhere in the body. Metastatic tumors are always malignant. The most common sources of metastatic tumors are cancers of the lung, breast, skin (melanoma), kidney and colon.

The treatment of malignant tumors to the brain is complex. Once a cancerous tumor spreads to the brain, there are several options for control or cure. These options include open surgery, whole brain radiation therapy, chemotherapy and radiosurgery. These options are often used in a combined approach to treatment.

Conventional whole brain radiation therapy has been used for more than 60 years to treat brain metastases. The entire brain is radiated using small doses of radiation, or fractions, each day for a number of weeks. This gradually builds up a dose large enough to effect tumor growth. Because the normal brain tissue is sensitive to radiation, the total dose given to the tumor is limited for whole brain radiation. Side effects are common: loss of hair, at one end of the spectrum of complications, and dementia and other brain injury at the other end of the spectrum of complications. Continued tumor growth and the development of new metastases are not unusual after whole brain radiation therapy because of dose limitations.

The concept and techniques of radiosurgery were developed by Professor Lars Leksell in Sweden many years ago. Radiosurgery avoids many complications and limitations of whole brain radiation therapy. Radiosurgery is more effective and convenient. Instead of radiating the whole brain, radiosurgery is specifically targeted treatment of the metastatic tumor. This means only the tumor receives a therapeutic dose of radiation, while the surrounding normal brain receives little or no toxic radiation. This also means the dose given to the tumor can be much greater (and effective) than the dose of radiation possible with whole brain radiation.

Radiosurgery is usually carried out in a single fraction on an out-patient basis. Patients can return to normal activities the day following Gamma Knife radiosurgery. Unlike whole brain radiation therapy, radiosurgery can be repeated should additional metastases appear or the original tumor continues to grow. We expect that 90% of metastatic tumors treated with radiosurgery will show no further growth, and many or most tumors will disappear.

Radiosurgery can be performed by linear accelerator systems adapted to radiosurgery, proton beam systems, and the Leksell Gamma Knife. Dr. Leksell actually envisioned and developed all of these methods of treatment, but ultimately selected the Gamma Knife because of its accuracy, effectiveness, and ease of use. It is the only radiosurgical device solely dedicated to use for tumors of the brain. (Other systems are adaptations of machines used for conventional radiotherapy.) Its design of focused sources of gamma rays, and fixed frame-based radiation delivery define the standard for radiation accuracy. This is critical for small tumors near important neural structures.

The value of Gamma Knife radiosurgery in the effective treatment of metastatic tumors for each patient at our Center is determined by our experienced team of neurosurgeons, neurotologists, radiation oncologists, physicists and nurses acting together, who have extensive experience in all aspects of tumor treatment, including open surgery, radiation therapy, and radiosurgery. Our Gamma Knife Center has treated thousands of patients and continues as the premier radiosurgical treatment resource for metastatic tumors in Greater San Diego.

An acoustic neuroma, also referred to (more accurately) as a vestibular schwannoma, is a benign tumor of the vestibular nerve that begins within the base of the skull and slowly expands into the skull cavity. Acoustic neuromas arise from the Schwann cells, which are responsible for the myelin sheath in the peripheral nervous system. The acoustic neuroma appears near the inner ear and their symptoms occur progressively in most patients. Some of these symptoms include slow and progressive destruction of hearing in the affected ear, a sense of imbalance and an altered gait, vertigo, nausea and vomiting. Interestingly, there is minimal or no tumor growth in some individuals.

If an acoustic neuroma does grow large enough, however, it can lead to increased intracranial pressure, which is associated with severe headaches and altered consciousness. The adjacent brainstem may also be compressed by these larger tumors, which affects a variety of local cranial nerves. The nearby facial nerve may also be affected. An acoustic neuroma that pushes against this nerve can lead to such symptoms as facial weakness and decreased facility of facial movements, facial numbness, sensory impairment, taste loss and the suppression of glandular secretions.

In the United States alone there are approximately 3,000 cases of acoustic neuroma diagnosed every year. Acoustic neuromas makes about 5 to 10 percent of all intracranial neoplasms in adult patients. Peak incidence of acoustic neuromas occurs during the fifth and sixth decades of a patient’s life and both men and women are affected equally. In acoustic neuroma treatments, Gamma Knife surgery has proven exceptionally effective. Over the past 25 years, thousands of patients suffering from vestibular schwannomas have been successfully treated by means of the Gamma Knife and the results compare favorably with the published results of microsurgery.

While microsurgery is currently the only method of physically removing a vestibular schwannoma, it involves much higher risks than Gamma Knife procedures. In the treatment of acoustic neuromas, Gamma Knife surgery avoids the need for actual incisions. There is minimal discomfort and no post-treatment recovery period. While Gamma Knife treatments do not entail the physical removal of an acoustic neuroma, they do enable doctors to control, halt, and sometimes even reverse the growth of the tumor. With Gamma Knife, there is low risk of cranial nerve complications.

Reports of re-operation on individuals treated with the Gamma Knife being more difficult or dangerous are unsubstantiated. Re-operation is quite rare and failure of control may be retreated by the use radiosurgery. Radiosurgery is an effective and safe tool in vestibular schwannoma treatment. Arguments that radiosurgery treatments may lead to cancer in the affected tumor are also unfounded, as there are no reports of cancer in a tumor being caused by radiosurgery.

Trigeminal neuralgia, also known as tic douloureux, is a neuropathic disorder caused by the compression of one or both of the patient’s trigeminal nerves. The trigeminal nerve is the fifth cranial nerve and it is commonly referred to as “the fifth nerve” or simply “V”. It is responsible for the perception of sensation in the face, and while it is primarily a sensory nerve, it is also involved in certain motor functions like chewing and swallowing. Studies have estimated that approximately 1 in 15,000 people suffer from trigeminal neuralgia. The disease tends to occur with greater frequency in patients who are 50 years or older, and it tends to be more common amongst elderly females. However there have been cases of trigeminal neuralgia in younger patients, some as young as three years of age.

The symptoms associated with trigeminal neuralgia are sharp, intensely mind-numbing shock-like stabs of pain in the face. These can occur separately or at once in different areas, like the ear, eye, nose, lips, forehead, scalp, teeth and side of the face. The pain usually occurs in one side of the face, depending on which trigeminal nerve is affected. However, there are some cases in which both of the patient’s trigeminal nerves are affected, resulting in bilateral pain. Pain can be triggered by interior or exterior stimuli. Even something as small as a chewing motion or a light finger’s touch against the cheek can trigger an episode of intense pain in an affected patient. At times, however, episodes of pain can occur without any apparent trigger.

The pain can last for a few seconds, a few minutes or several hours. Loud noises and large crowds, talking, chewing, and even (in the worst cases) the making of facial expressions can aggravate the condition. Numbness is not usually associated with trigeminal neuralgia unless there is co-existing multiple sclerosis. Pain associated with trigeminal neuralgia occurs in cycles, and often there may be spontaneous remissions from pain lasting weeks to years. Interestingly, this pain usually responds to carbamazepine (Tegretol), an oral anticonvulsant medication.

Trigeminal neuralgia is usually caused by compression of the sensory (trigeminal) nerve within the skull by a small artery or vein at the point where the nerve joins the brain stem. Sometimes a small, benign tumor compresses the nerve, causing jolts of electrical shock-like pain to radiate into the face. A small percentage of trigeminal neuralgia patients also suffer from multiple sclerosis. In this case the inflammatory response affecting the brain also involves the trigeminal nerve, causing paroxysmal pain.

Tic douloureaux is unique among pain disorders because nearly all treatments work for a period of time. Over the years, peripheral nerve avulsion, heating, cooling, compressing, decompressing, chemical ablation, and irradiation have all enjoyed varying degrees of success. Because of the effectiveness of carbamazepine (Tegretol), its use is usually the first level of treatment. Other anticonvulsants may be tried, but these are not usually as effective. When oral medication fails to control this dreadful pain, surgical measures must be taken. Among these, trigeminal neuralgia radiosurgery has proven to be especially effective and safe.

With the use of the Gamma Knife, trigeminal neuralgia can be successfully treated without the need for surgical incisions that might result in further complications. A single, non-invasive morning trigeminal neuralgia treatment has resulted in excellent pain relief in 58% of cases, good pain relief in 36% and failed pain relief in 6%. Transient facial numbness is rare. Long term recurrence rates are unknown. This treatment is a suitable alternative to anticonvulsant therapy and compares favorably to other trigeminal neuralgia treatments. Trigeminal neuralgia radiosurgery is an especially preferable treatment option for patients who are medically unfit for the use of general anesthetic.

Meningiomas are the most common benign brain tumors. Meningiomas account for 20% of all brain tumors and about 25% of all primary spinal cord tumors. The brain is protected by three layers of tissue called the meninges. The outer layer is composed of thick dura mater, where meningiomas begin growth. The middle layer is called the arachnoid and the innermost layer is known as the pia mater.

Meningiomas are more common in women than in men. While meningiomas affect people of all ages, they are most common among people in their forties. Meningiomas generally grow slowly, usually do not invade surrounding normal tissue and rarely spread to other parts of the central nervous system or body.

Gamma Knife is a primary method for the control of meningiomas. Radiosurgery can be the initial treatment for difficult to operate skull-based tumors or in the treatment of tumors recurring after open surgery. Skull-based meningiomas frequently recur after operation and conventional surgery may occasionally lead to increased cranial nerve dysfunction. Tumors arising from the cavernous sinus, and petroclival tumors of the posterior fossa are especially good candidates for Gamma Knife radiosurgery, as complications of complex, skull base surgery are avoided. Gamma Knife has a 95% control rate for the treatment of meningiomas.

Astrocytoma is defined as a tumor composed of astrocytes. It is the most common type of primary brain tumor and is also found throughout the central nervous system. One classification groups astrocytomas according to their histologic appearance and distinguishes pilocytic, protoplasmic, gemistocytic, and fibrillary types. Another classification groups them in order of increasing malignancy as Grade I, Grade II, Grade III, and Grade IV astrocytomas.

Glial tumors may cause patients to develop many symptoms. A more benign type of glioma can occur in younger people and could present as a seizure. Depending on the area of the brain involved, a progressive neurological problem, such as weakness, numbness, or speech problems can develop. Since the more malignant tumors enlarge rapidly, symptoms of increased pressure in the head are common: headache, visual loss, and personality change. Headache is characteristically worse in the morning when awakening. On rare occasions a glial tumor can bleed spontaneously, presenting with an acute neurological deficit of stroke.

The classification of gliomas is based upon the appearance of these tumors under the microscope. This requires a biopsy of tumor tissue and in general is predictive of the behavior of the tumor and the outlook for the patient. Glial tumors are divided into two basic cell types: astrocytomas and oligodendrogliomas. The most common grading system for astrocytomas is the following World Health Organization (WHO) system:

Grade I: Pilocytic Astrocytoma – a group of generally slow-growing astrocytomas, including most fibrillary and pilocytic astrocytomas.

Grade II: Fibrillary Astrocytoma – astrocytomas with slightly more malignant potential than those of Grade I, including astroblastomas and some fibrillary and pilocytic astrocytomas. One classification system includes some anaplastic astrocytomas in this group.

Grade III: Anaplastic Astrocytoma – moderately malignant astrocytomas, sometimes including the less malignant of the gliosblastoma multiforme group.

Grade IV: Glioblastoma Multiforme – astrocytoma that is highly malignant. This group includes only the glioblastoma multiforme type, although some less malignant glioblastomas are sometimes classified as Grade III.

Stereotactic radiosurgery may be used to treat gliomas deep within the brain, near sensitive brain regions or when the patient is not a candidate for surgery.

An arteriovenous malformation (AVM) is an unusual formation of arteries and veins which are congenital in origin and occur throughout the body. When they occur within the brain they may cause symptoms such as seizure, headache or progressive neurological deficits. Most importantly they can spontaneously bleed resulting in a stroke with lasting neurological problems or even death.

An AVM may have several forms, such as a direct connection between an artery and vein, an AV fistula. Unusual formations of veins which bleed and cause seizures are cavernous angiomas. Abnormalities of very small vessels are capillary angiomas. The most important (and dangerous) are AVM’s which have both arterial and venous components. These AVM’s have a 3 to 4 percent chance of spontaneous hemorrhage each year. Roughly 10 percent of the hemorrhages will be fatal and about 15 percent of victims will suffer a continuing neurological deficit, such as weakness, sensory or visual loss, speech abnormality, etc.

The gold standard for diagnosis is a cerebral angiogram. The radiologist advances a catheter into the arteries which supply the brain and images the AVM nidus by injecting radio-opaque dye with serial x-rays of the skull. MR techniques can also identify AVM’s.

Natural history
It is important to understand the natural history of AVM’s since this impacts the advise given to patients who harbor unruptured and asymptomatic malformations. Surgery can cure AVM’s, but not all AVM’s are amenable to open operation because of the risk. This risk is generally predictable from the size and position of the AVM. The age and general health of the patient also factor into the pre-operative equation. An important contribution to the estimation of pre-operative risk was the Spetzler-Martin grading system. This paradigm assigns points according to AVM size, position in eloquent brain (important functioning) and the presence of deep, draining veins. The higher the score, the greater the risk of post-operative problems.

Surgical excision of the AVM brings an immediate cure, but not all AVM’s can safely be removed. Gamma Knife radiosurgery can play a role in the obliteration of smaller AVM’s. This technique has the advantage that is non-invasive and accomplished in a single session. The disadvantage is that the effects of focused radiation occur over months and years, during which time the patient is still at risk for spontaneous hemorrhage. In general, AVM’s under 1 cm in diameter have a 90 percent chance of obliteration, while those under 2 cm have an 80 percent change of cure. Only about 50 percent of malformations 3 cm in size are obliterated.

Interventional radiology can reduce the blood supply to AVM’s and rarely cure them. This technique is performed by selectively filling the feeding arteries, or veins with clotting agents by a catheter during angiography. Endovascular therapy usually plays a supporting role, reducing the blood supply to aid open surgery or to reduce the size of the AVM before Gamma Knife radiosurgery.

Once an AVM has ruptured, surgery is indicated to prevent repeat hemorrhage. Most experts advise emergency operation for patients in danger of death from very large clots. Smaller, minimally symptomatic or asymptomatic bleeds can be managed medically, as the hematoma is gradually reabsorbed from the brain tissue. Early re-bleeding is rare in AVM’s (6 percent in the first 6 months after initial hemorrhage and there is no proof early surgery can reduce the eventual outcome in individuals with a deficit, since the brain damage has already occurred.

Glioblastoma (GBM) is a primary brain tumor arising from the glial tissue which nourishes and supports the brain. There are several different types of glial cells, astrocytes, oligodendrocytes and ependymal cells. Gliomas are the most commonly diagnosed of both benign and malignant primary brain tumors. Glial tumors account for 45-50% of all primary brain tumors. The most common gliomas are astrocytomas, ependymomas, oligodendrogliomas and tumors with mixtures of two or more of these cell types.

One of the many protocols for the treatment of glioblastoma is surgical intervention followed by chemotherapy. A post surgery MRI scan may indicate a radiosurgical boost or conventional radiation therapy. Also, a recurrence may be treated with Gamma Knife radiosurgery. The following study demonstrates the role of radiosurgery in the treatment of malignant gliomas.

A University of Pittsburgh study revealed that patients treated with a Gamma Knife boost following surgery or biopsy, and patients treated at recurrence of disease, roughly 6 months after initial treatment showed improved survival benefit from Gamma Knife radiosurgery. Survival after treatment with first recurrence of GBM was somewhat better (30 months) than initial boost treatment (20 months). About 1 in 5 patients with GBM required additional surgery after Gamma Knife radiosurgery, for tumor recurrence rather than tumor necrosis. The 2-year survival rate for GBM was 51%. Individuals with anaplastic astrocytoma fared better with a median survival of 32 months and 2 year survival of 67%. The tumors treated with radiosurgery were small, 3 cm. (Kondziolka D, et al., J. Neurosurg. 1997;41:776-785).

The pituitary is a small, pea-sized gland that hangs from the hypothalamus, a structure at the base of the brain, by a thread-like stalk that contains both blood vessels and nerves. It controls a system of hormones in the body that regulate growth, metabolism, the stress response, and functions of the sex organs via the thyroid gland, adrenal gland, ovaries, and testes.

A pituitary tumor is an abnormal growth of cells within the pituitary gland. Most pituitary tumors are benign, which means they are non-cancerous, grow slowly and do not spread to other parts of the body. However, these cells can make the pituitary gland produce too many hormones, which can cause problems in the body. Tumors that make hormones are called functioning tumors, and they can cause a wide array of symptoms depending upon the hormone effected. Tumors that don’t make hormones are called non-functioning tumors.

As tumors enlarge, normal pituitary function is destroyed. This produces various hormonal deficiencies, since the pituitary controls the action of other endocrine glands. Pressure on near-by structures produce headaches, vision problems, nausea, and vomiting. The optic nerves are directly above the pituitary gland and upward growth of pituitary tumors frequently causes progressive visual loss. This visual loss typically begins from each side of the field of vision leading to tunnel vision.

Pituitary tumors often go undiagnosed because their symptoms resemble those of many other common diseases.

Gamma Knife radiosurgery is usually prescribed for a patient left with a small amount of residual tumor following surgical removal. This is particularly important for the patient who has persistent excessive hormone production. Gamma Knife radiosurgery may also be used to treat a small residual non-functioning tumor, particularly if it has recurred. Gamma Knife is usually considered as “adjunctive therapy” – given after surgical removal of as much tumor as possible. The size of the residual tumor is the limiting factor in selecting a patient for this treatment – the tumor cannot be too close to the optic chiasm (eye nerves) because of the risk of damage to vision. A recent MRI study must be reviewed before deciding if the patient is a candidate for Gamma Knife.

Gamma Knife radiosurgery is suitable for the treatment of pituitary adenomas (a benign tumor of glandular origin). Further, treatment given in a single session is more convenient for the patient and is cost effective.

Research Studies
Metastic Brain Tumors
Gamma Knife surgery for the treatment of 5 to 15 metastases to the brain

DaviD J. Salvetti, B.e., tara G. NaGaraJa, B.S., iaN t. McNeill, M.S., ZhiyuaN Xu, M.D., aND JaSoN SheehaN, M.D., Ph.D.

Conclusions. In patients with 5–15 brain metastases at presentation, the number of lesions did not predict sur- vival after GKS; however, the RPA class was predictive of OS in this group of patients. Gamma Knife surgery for such patients offers an excellent rate of local tumor control.

A Phase 3 Trial of Whole Brain Radiation Therapy and Stereotactic Radiosurgery Alone Versus WBRT and SRS With Temozolomide or Erlotinib for Non-Small Cell Lung Cancer and 1 to 3 Brain Metastases: Radiation Therapy Oncology Group 0320

Paul W. Sperduto, MD, MPP,* Meihua Wang, PhD,y H. Ian Robins, MD, PhD,z Michael C. Schell, PhD,x Maria Werner-Wasik, MD,jj Ritsuko Komaki, MD,{
Luis Souhami, MD,# Mark K. Buyyounouski, MD,** Deepak Khuntia, MD,yy
William Demas, MD,zz Sunjay A. Shah, MD,xx Lucien A. Nedzi, MD,jjjj Gad Perry, MD,{{ John H. Suh, MD,## and Minesh P. Mehta, MD***


Patients with non-small cell lung cancer and 1 to 3 brain metastases were randomly assigned to whole brain radiation therapy (WBRT) þ stereotactic radiosurgery (SRS), WBRT þ SRS þ temozolamide, or WBRT þ SRS þ erlotinib. The median survival times for the 3 arms were 13.4, 6.3, and 6.1 months, respectively. Grade 3 to 5 toxicity was 11%, 41%, and 49%, respectively. Possible explanations for these findings are discussed.

Arteriovenous Malformation (AVM)
Stereotactic radiosurgery for arteriovenous malformations, Part 1: management of Spetzler-Martin Grade I and II arteriovenous malformations

Hideyuki kano, M.d., PH.d.,1,3 L. dade Lunsford, M.d.,1–3 JoHn C. fLiCkinger, M.d.,1–3 Huai-CHe yang, M.d.,1,3,4 THoMas J. fLannery, M.d., PH.d.,1,3 nasir r. awan, f.C.P.s.,1,3 aJay niranJan, M.CH., M.B.a.,1,3 Josef novoTny Jr., PH.d.,2,3
and dougLas kondzioLka, M.d.1,3

Conclusions. Stereotactic radiosurgery is a gradually effective and relatively safe option for patients with small- er volume Spetzler-Martin Grade I or II AVMs who decline initial resection. Hemorrhage after obliteration did not occur in this series. Patients remain at risk for a bleeding event during the latency interval until obliteration occurs. Patients with aneurysms and an AVM warrant more aggressive surgical or endovascular treatment to reduce the risk of a hemorrhage in the latency period after SRS.

Analysis of nidus obliteration rates after gamma knife surgery for arteriovenous malformations based on long-term follow-up data: the University of Tokyo experience


Conclusions. After the introduction of CT and MR images into dose planning, the conformity and selectivity of dosime- try improved remarkably, although the latency intervals until obliteration were prolonged. Imaging outcomes for AVMs should be evaluated using data provided by longer follow-up periods. The timing of additional treatments for residual AVMs should be decided cautiously, considering the size of the AVM, the patient age and sex, and the history of hemor- rhage before radiosurgery.

Trigeminal Neuralgia

Michele Longhi, M.D., Paolo Rizzo, M.D., Antonio Nicolato, M.D., Roberto Foroni, Ph.D., Mario Reggio, Ph.D., Massimo Gerosa, M.D.

CONCLUSION: According to our experience, GKR represents a reliable second-line therapeutic approach for TN after pharmacological failure. Favorable prognostic fac- tors include “primary GKR” and maximal GKR dose ranging between 80 and 90 Gy.

Fifteen years of Gamma Knife surgery for trigeminal neuralgia in the Journal of Neurosurgery: history of a revolution in functional neurosurgery

Jean Régis, M.D., anD Constantin tuleasCa, M.D.

In conclusion, this series of the Top 25 papers in JNS on GKS for trigeminal neuralgia bears witness to the fact that radiosurgery is an example of a true disruptive in- novation in the eld of functional neurosurgery and, spe- ci cally, in the neurosurgical management of trigeminal neuralgia. These articles demonstrate how greatly this innovation has changed neurosurgical practice in just a few years.

Single-fraction radiosurgery of benign cavernous sinus meningiomas

Bruce e. Pollock, M.D.,1,2 Scott l. StafforD, M.D.,2 Michael J. link, M.D.,1 YolanDa i. GarceS, M.D.,2 anD roBert l. foote, M.D.2

Conclusions. Single-fraction SRS at the radiation doses used in this series provided durable tumor control for patients with benign CSM. Larger tumor volume remains the primary factor associated with complications after single-fraction SRS of benign CSM despite advancements in SRS technique.



Conclusions: SRS is an effective and safe treatment for CSM, feasible either in the primary or the postsurgical setting. Incomplete coverage of the target did not worsen outcomes. More than 80% of the patients remained free of disease progression during long-term follow-up.

Acoustic Neuroma


Conclusions: SRS is an effective and safe treatment for CSM, feasible either in the primary or the postsurgical setting. Incomplete coverage of the target did not worsen outcomes. More than 80% of the patients remained free of disease progression during long-term follow-up.

Early Radiosurgery Improves Hearing Preservation in Vestibular Schwannoma Patients With Normal Hearing at the Time of Diagnosis

Berkcan Akpinar, BA,* Seyed H. Mousavi, MD,y Michael M. McDowell, MD,y Ajay Niranjan, MD,y Amir H. Faraji, MD, PhD,y John C. Flickinger, MD,z and L. Dade Lunsford, MD

Conclusions: SRS within 2 years after diagnosis of VS in normal hearing patients re- sulted in improved retention of all hearing measures compared with later SRS.

Faculty Publications
Stereotactic Radiosurgery Treatment of Trigeminal Neuralgia: Clinical Outcomes and Prognostic Factors

Zachary J. Taich1, Steven J. Goetsch2, Elsa Monaco2, Bob S. Carter1, Kenneth Ott2, John F. Alksne1, Clark C. Chen1

CONCLUSIONS: Excellent TN pain relief was achieved with the delivery of 85 Gy in a single-shot, 4-mm isocenter SRS targeting the dorsal root entry zone. Patients with classical TN, with age older than 70 years, or who under- went previous percutaneous procedures were more likely to benefit from SRS. SRS is efficacious in patients with classical TN despite concurrent diagnosis of MS.


* San Diego Gamma Knife Center, Grossmont Cancer Center, La Jolla, CA; y Department of Medical Physics, San Diego State University, La Jolla, CA

The remaining challenges for this now well-accepted field include:

(1) Continuing to develop training programs for new radio- therapy centers beginning to develop SRS and stereotac- tic radiotherapy programs.

(2) Updating QA guidelines to account for new develop- ments in SRS and stereotactic radiotherapy, including computer-controlled equipment and automatic position- ing devices.

(3) Writing new dosimetry protocols to account for the novel geometries used in SRS systems.

(4) Developing protocols for accurate and appropriate stereotactic image acquisition and registration.