Stereotactic radiation therapy
Stereotactic radiation therapy is a type of external beam radiation therapy. Stereo means 3-dimensional (3-D) and tactic means to probe. Stereotactic radiation therapy allows radiation beams to be given to a very specific area, usually the brain.
There are 2 ways to deliver stereotactic radiation therapy treatment.
- Stereotactic radiosurgery (SRS) delivers a single high dose of radiation to the tumour (called a single fraction). This treatment doesn’t involve surgery. An incision (cut) is not made and tissue is not surgically removed.
- Stereotactic radiotherapy (SRT) gives smaller doses of radiation over a number of treatment sessions (called multiple fractions), until the desired total dose is given.
Stereotactic radiosurgery requires specialized equipment and training, so it may not be available at all cancer treatment centres in Canada.
Stereotactic radiation therapy is mainly used to treat small brain tumours that are either malignant or benign. It is used to treat:
- tumours deep within the brain
- tumours that are hard to reach or that cannot be removed by surgery because doing so would damage too much normal brain tissue
- a recurrence or metastasis in the brain
Stereotactic radiation may also be used to give an additional dose of radiation (boost) to the brain after conventional external beam radiation therapy to kill any remaining cancer cells.
Stereotactic radiation therapy requires complex treatment planning using computers and high-energy radiation machines. A computed tomography (CT) scancomputed tomography (CT) scanAn imaging technique that uses a computer to put a series of x-ray images together to create a 3-dimensional picture of organs, tissues, bones and blood vessels inside the body. A contrast medium may be injected to make organs and structures show up clearly on the x-ray images. or magnetic resonance imaging (MRI)magnetic resonance imaging (MRI)An imaging technique that uses a magnetic field to produce pictures of areas inside the body. A contrast medium may be injected into the body to make structures and organs show up clearly on the image. is used to find and map the exact area to be treated. This helps ensure that the surrounding normal tissue is spared from radiation.
When treating brain tumours, the head must be kept in the same position for each treatment. People receiving stereotactic radiation therapy to the brain wear a special head frame (stereotactic frame) during their treatment.
- When SRS is used, the head frame is attached to the skull. A neurosurgeon (a doctor who specializes in brain and nervous system surgery) injects a local anesthetic just under the scalp to numb the area. Then the surgeon places special screws or pins into the skull. These screws (or pins) fix the head frame to the skull. The skull screws and head frame are removed after the treatment is finished.
- When SRT is used, a removable head frame or a special face mask keeps the head in position.
Stereotactic radiation delivery systems include:
The Gamma Knife is not a knife. It is a machine that precisely focuses about 200 beams of gamma radiation (usually cobalt) at a tumour. The radiation is delivered at the point where the beams cross or intersect. It is usually given as a single high-dose treatment. The technique is sometimes called Gamma Knife surgery (GKS).
Linear accelerator (LINAC)
The LINAC machine delivers radiation in several x-ray beams that follow arcs. The treatment may be given over several sessions. Each session may take 30 minutes or longer.
CyberKnife is an advanced type of linear accelerator. A robotic system points the linear accelerator in a variety of positions. Several x-ray cameras (or imaging devices) and computers are used to track the person’s position. If a person moves slightly, the robotic system can adjust by repositioning the linear accelerator before the beam of radiation is delivered. This type of treatment may need to be given over a period of days.
The use of CyberKnife is not limited to brain tumours. It can be used for tumours in other parts of the body, such as the spine or lung. Small, local tumours seem to benefit most from this treatment. A stereotactic frame is not needed, but other immobilization devices or special markers may be used.