BACKGROUND
Stereotactic radiosurgery is the precise delivery of high-dose radiation to the brain or spine, can be used to treat a variety of neurological diseases, and is usually administered in a single or short treatment course (it is differentiated from radiotherapy, which is fractionated therapy given over one to several weeks). The concept of radiosurgery is to spare neighboring normal tissue and focus radiation on the abnormal tissue, or pathology, minimizing the side effects and risks.

SIGNS and SYMPTOMS of DISEASE
A neurosurgeon may use radiosurgery as part of a program to treat conditions such as primary brain tumors (benign and malignant), metastatic tumors (cancer spread from other regions of the body), vascular malformations (abnormal blood vessels), or functional disorders (e.g. the facial pain condition known as trigeminal neuralgia). Each of these processes would come to attention differently: for example tumors could present with progressive headaches, weakness, or seizures, whereas vascular malformations could present with an abrupt debilitating headache because of bleeding.

DIAGNOSTIC TESTS
Imaging techniques such as CT and MRI are used to accurately characterize the size and shape of the lesion, and are essential in the design of an effective and safe plan that will maximally radiate the intended target and spare normal adjacent tissue.

An example of a treatment plan to focus radiation at the lesion:
  

TREATMENT OPTIONS

Radiosurgery is sometimes used as an upfront, or first-line therapy, or can also be used in conjunction with other methods such as surgery and chemotherapy. In addition, radiosurgery can be combined with other types of radiotherapy, to boost treatment to a recurrent or residual lesion. The choice of treatment protocol will depend on the type of disease treated, its location, and the individual patient’s circumstances.

However, not all cases are appropriate for radiosurgery, each case should be evaluated by a neurosurgeon active in stereotactic radiosurgery. For example, while many tumors can and should be addressed with surgery, some tumors are less accessible and better treated with radiosurgery.

TECHNIQUE
Several forms of stereotactic radiosurgery are now available. The most widely used types are known as linear accelerator, (LINAC), and cobalt-60 or photon (gamma wavelength) based, known by the brand name Gamma Knife. The other available form of radiosurgery is particle or proton beam therapy, and is in limited use nationwide. Each instrument type is best suited for particular pathologies and disease locations.

LINAC treatment can be delivered by several different machine types with brand names such as X-Knife, Synergy, Trilogy, Novalis, and CyberKnife. These devices deliver high energy X-rays from a moving gantry, or machine arm, to the designated lesion. Larger lesions can be treated with LINAC by fractionating, or dividing, the treatment into multiple sessions. A mask holds the patient in the proper position during the treatment and ensures no movement, which would misdirect the radiation.

Gamma Knife treatment utilizes a frame that is affixed to the patient’s head and skull to ensure no movement during treatment. Multiple radioactive sources are directed through a helmet to the target in a single treatment. The patient and machine are fixed in place during treatment.

LINAC (Novalis) machine

Gamma Knife machine

SURGICAL RISKS
All radiation treatment is geared toward destruction of the target. As the targeted lesion dies, it may induce edema, or swelling, at the target and in nearby tissues. This may produce headaches, lethargy, weakness and/or numbness, and is usually treated with a short course of steroids. The specific symptoms experienced will be dependent on the location of the nervous system affected. For example, edema near the optic nerves, or the nerves that connect the eyes with the brain, would affect vision, where as edema near other pathways involved in coordinating movement could present as weakness. In addition, as targeted tissue dies, it can also undergo ‘radiation necrosis,’ a type of tissue death that can be permanent and very rarely can require surgical removal.

EXPECTED OUTCOME
The radiation delivered affects the genetic material, or DNA, of the target lesion. Depending on the type of lesion, radiation can cause errors in tumor cell reproduction, halting growth, or affect the blood vessels that supply and support the lesion. While the radiation itself does not remove the lesion, it can cause a gradual shrinking of the lesion, which can take some time to occur (months to years).

AUTHOR
Hugh Moulding, MD PhD, James J Evans, MD