Radiation therapy uses high energy x-rays or particles to kill tumor cells. This type of treatment is given by a doctor called a radiation oncologist. Radiation therapy may be recommended if surgery is not an option, or when pituitary tumors persist or come back after surgery, cause symptoms that are not relieved by medicines, have a large and irregular shape, or are very close to the optic nerves.
Treatment may be given with conventional radiation therapy or with other techniques such as stereotactic radiosurgery.
Conventional radiation therapy
In conventional radiation therapy, radiation is directed at the pituitary from a source outside the body. It is much like getting an x-ray, although the doses of radiation used are much higher.
Before your treatments start, the radiation team will take careful measurements to determine the correct angles for aiming the radiation beams and the proper dose of radiation. The radiation is then given in a series of treatments, usually 5 times a week over 4 to 6 weeks. At each session, you lie on a special table while a machine delivers the radiation from a precise angle. The treatment is not painful. Each session lasts about 15 to 30 minutes. Much of that time is spent making sure the radiation is aimed correctly. The actual treatment time each day is much shorter.
This therapy is effective but has some disadvantages. The first is that it works slowly. It can take months or years before the tumor growth and/or hormone production is fully controlled. Also, this therapy cannot avoid damaging the remaining normal pituitary. In most cases, normal pituitary function will be lost over time. This approach also may damage some normal brain tissue, particularly near the pituitary gland, which may result in a decrease in mental function years after therapy. The optic nerves may also be damaged, resulting in impaired vision. The radiation treatment may also increase the risk of developing a brain tumor later in life, although this risk is still likely to be low.
These risks may be lower with the use of newer techniques that focus the radiation more precisely on the pituitary, such as intensity modulated radiation therapy (IMRT), stereotactic radiation, and proton beam therapy. The use of these techniques may be limited for some tumors that are very close to the optic nerves.
Intensity modulated radiation therapy (IMRT)
IMRT is an advanced form of three-dimensional radiation therapy. It uses a computer-driven machine that actually moves around the patient as it delivers the radiation. IMRT allows the doctor to shape the radiation beams and aim them at the tumor from several angles. The intensity (strength) of the beams can also be adjusted to minimize the dose reaching the most sensitive nearby normal tissues. This may result in fewer side effects. Many major hospitals and cancer centers now use IMRT.
Stereotactic radiosurgery/stereotactic radiation therapy
This type of treatment delivers a large, precise radiation dose to the tumor area in a single session (radiosurgery) or in a few sessions (radiotherapy). (There is no actual surgery involved in this treatment.) It targets the tumor more precisely, causing less harm to the remaining normal pituitary gland, and avoids exposing the rest of the brain to radiation.
For this treatment, a head frame is attached to the skull to help precisely aim the radiation beams. Once the exact location of the tumor is known from CT or MRI scans, radiation may be delivered in one of two ways.
In one approach, radiation beams from a machine are focused at the tumor from hundreds of different angles for a short period of time. An example of such a machine is the Gamma Knife.
Another approach uses a movable linear accelerator (a machine that creates radiation) that is controlled by a computer. Instead of delivering many beams at once, this machine moves around the head to deliver radiation to the tumor from different angles. Several machines do stereotactic radiosurgery in this way, with names such as X-Knife, CyberKnife, and Clinac.
Stereotactic radiosurgery typically delivers the whole radiation dose in a single session, though it may be repeated if needed. Sometimes doctors give the radiation in several treatments to deliver the same or a slightly higher dose. This is called fractionated radiosurgery or stereotactic radiotherapy.
Like conventional radiation therapy, the benefit of stereotactic radiation develops slowly. It may take months to be fully effective.
Unfortunately, this therapy cannot be used for tumors that are very close to important nerves near the pituitary, such as those needed for vision. Its use is also limited in cases of unusual tumor shape.
Proton beam radiation therapy
This form of treatment uses a beam of protons rather than x-rays to kill cancer cells. Protons are positive parts of atoms. Unlike x-rays, which release energy both before and after they hit their target, protons cause little damage to tissues they pass through and then release their energy after traveling a certain distance. Doctors can use this property to deliver more radiation to the tumor with less damage to nearby normal tissues. Like stereotactic radiation, it has the advantage of focusing the radiation more precisely on the pituitary tumor.
But proton beam radiation therapy requires highly specialized equipment and is not widely available – there are only a handful of proton beam centers in the United States at this time. It is not a standard treatment for pituitary tumors. Studies are still needed to see if it is safer or more effective than stereotactic radiosurgery.
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