Stereotactic radiosurgery and radiotherapy refer to the precise delivery of radiation to targeted areas, such as brain tumors or AVM's (abnormal tangles of blood vessels). They differ from normal radiation regimes in that very little, if any, normal tissue is exposed to the radiation.

Basically, the difference between stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) has to do with the intensity and duration of the radiation treatments. 

Stereotactic radiosurgery (SRS)

Stereotactic radiosurgery (SRS) delivers radiation at a very high intensity, all at once, to a small area. It is a way of treating brain disorders with a precise delivery of a single high dose of radiation in a one-day session.

Stereotactic radiotherapy (SRT)

Treating over time is called "fractionated stereotactic radiotherapy" or (SRT).  With stereotactic radiotherapy (SRT), the radiation is given at different times, at lower intensities and to larger areas.   One benefit of this technology is its ability to easily treat very large tumor volumes by treating over time during cell division.

Different technologies

Currently, there are three different technologies that are used for stereotactic radiosurgery and/or stereotactic radiotherapy. Each instrument operates differently, has a different source of radiation, and may be more effective under different circumstances.

1. Cobalt60 based (photon) - Gamma Knife
2. Linear accelerator based (photon).
3. Particle beam (proton)

1). The Gamma Knife is a cobalt60 based machine that provides extremely accurate targeting and precise treatment for brain cancers.  The Gamma Knife does not move during treatment, thus providing a high degree of precision within the brain. It also utilizes multiple sources of radiation, which allows for less damage to healthy tissue and better targeting. These machines are ideal for smaller tumors and lesions and for functional disorders of the brain. The UW Medicine Gamma Knife Center is located at Harborview Medical Center.

2). The linear accelerator has the ability to perform radiosurgery on larger tumors by delivering treatments over several days, which is referred to as "radiotherapy."  This is a flexibility not available with other machines.  The linear accelerator uses one large, intense radiation beam that is redirected in many "arcs" to lessen the effect on healthy tissue. These machines can be utilized throughout the body as well as the head and neck.  

3). The particle beam or cyclotron treats brain tumors and body cancers in a fractionated manner (SRT). There is not much material available on the technology and outcomes at this time.

Following is an overview of a gamma knife procedure. The Gamma Knife administers stereotactic radiosurgery. This is because Gamma Knife surgery requires precise positioning, special equipment is needed. A specialized helmet and frame are fitted to the patient’s head and serves two functions:

• Helps aim the Gamma Knife at the right location in the brain, and; 
• Keeps the head steady and properly positioned during the procedure  (figure 1)

Image Mapping and Planning

Once the target area is located, imaging software is used to create three-dimensional models of the brain. These models closely match the shape of the tumor or the affected area. Then a prescription dose is determined. This dose will be dependent upon the type of lesion, location, size and any prior therapy. Planning generally takes about 30 minutes.

Treatment

The doctor may advise you not to eat or drink anything after midnight the night before the Gamma Knife therapy. It is also advisable not to wear jewelry or other extra items the day of the procedure.

Treatment is completed in a single session. Depending upon the treatment plan and how many radiation shots the patient requires (usually 1-4), the procedure is generally completed in less than an hour.

During treatment, the patient is monitored by means of a video system and a two-way intercom that allows the patient to talk with the physician during the procedure. When the procedure is completed, the head frame is removed and the patient is evaluated and prepared for discharge. Many patients return to their normal activities the following day.

It may take many weeks or even up to several years before the effects of Gamma Knife treatment become apparent. The patient’s progress is monitored through follow-up imaging studies.

Because radiosurgery administers a higher intensity dose of radiation, it is not used to treat areas of the body other than the head. This is because there is no way to immobilize the body to treat a tumor in one session the way the head can be secured.  That is why radiosurgery is limited to the head and neck as these areas can be immobilized with skeletal fixation devices that completely restrict the head's movement, permitting the most precise and accurate treatment.

However, stereotactic radiotherapy (SRT), which administer lower-intensity doses of radiation over a period of days or weeks, may be administered in the body with the assistance of removable masks and frames that achieve some degree of lesser mobilization. 

In general, radiation does not remove the tumor or lesion, but it damages the DNA of the tumor cells. This damaged DNA eventually causes the cells to die, because they cannot reproduce or maintain their function.

On the other hand, radiation causes the blood vessels of lesions, such as AVM's, to thicken and close off. The shrinking of a tumor or closing off of a vessel occurs over a period of time and depends on the type of tumor or vascular lesion.