External Beam Radiation Therapy Treatment
The goal of radiation therapy is to direct large enough doses of ionizing radiation into the body to kill cancer cells while sparing the surrounding healthy tissue from damage. Several different radiation therapy techniques have been developed to accomplish this. Depending on the location, size and type of your tumor or tumors, you may receive one or a combination of these techniques. Your cancer treatment team will work with you to determine which treatment and how much radiation is best for you.
During external beam radiation therapy, beams of radiation are directed through the skin to a tumor and the immediate surrounding area to destroy the main tumor and any nearby cancer cells. To minimize side effects, the treatments are typically given every day for a number of weeks.
The radiation comes from a machine located outside of your body that does not touch your skin or the tumor. Receiving external beam radiation is similar to having an X-ray taken. It is a painless, bloodless procedure. The machine delivering the radiation is called a linear accelerator, sometimes called a 'linac.' It produces a beam of high-energy X-rays or electrons. Using sophisticated treatment planning software, your radiation oncology treatment team plans the size and shape of the beams, as well as how they are directed at your body, to treat your tumor effectively while relatively sparing the normal tissues surrounding the cancer cells.
Several special types of external beam therapy are discussed below. These are used for particular types of cancer, and your radiation oncologist will recommend one of these treatments if he or she believes it will help you.
Three-Dimensional Conformal Radiation Therapy (3D-CRT)
Tumors usually have an irregular shape. Three-dimensional conformal radiation therapy (3D-CRT) uses sophisticated computers and computer assisted tomography scans (CT or CAT scans) and/or magnetic resonance imaging scans (MR or MRI scans) to create detailed, three-dimensional representations of the tumor and surrounding organs. Your radiation oncologist can then shape the radiation beams to the size and shape of your tumor. The tools used to shape the radiation beams are multileaf collimators or blocks. Because the radiation beams are very precisely directed, nearby normal tissue receives less radiation exposure.
Intensity Modulated Radiation Therapy (IMRT)
Intensity modulated radiation therapy (IMRT) is a specialized form of 3D-CRT that allows radiation to be shaped more exactly to fit your tumor. With IMRT, the radiation beam can be broken up into many "beamlets," and the intensity of each beamlet can be adjusted individually. Using IMRT, it may be possible to further limit the exact amount of radiation that is received by normal tissues that are near the tumor. In some situations, this may also allow a higher dose of radiation to be delivered to the tumor, increasing the chance of a cure. If three-dimensional RT is the equivalent of a high-quality photograph, IMRT is the equivalent of a high-quality oil painting, with added variable thickness of the paint.
Electron Beam Therapy
In some circumstances, tumors are sufficiently close to the skin that a less penetrating type of radiation can be used and spare the underlying tissues. Electrons have both mass and charge and therefore do not travel as far in tissue as an equivalently energetic X-ray. This type of treatment provides an advantage for patients who, for example, have breast cancers close to their skin surface and for thick tumors that arise in the skin.
Stereotactic radiotherapy is a technique that allows your radiation oncologist to use very small beams of radiation to destroy certain types of small tumors. Since the beam is so precise, your radiation oncologist may be able to deliver treatment more quickly than with other techniques. This additional precision is achieved through rigid immobilization, such as with a head frame as is used in the treatment of brain tumors. Treatment can usually be delivered in as few as 3-8 sessions. Stereotactic radiotherapy may be the only treatment needed if a very small area is affected. In addition to treating tumors, it can also be used to treat malformations in the brain's blood vessels and certain noncancerous (benign) brain tumors.
Image-Guided Radiation Therapy (IGRT)
In some facilities, such as ours, radiation oncologists use image-guided radiation therapy (IGRT) to help them better aim the radiation at the cancer. Normal structures and tumors can move between daily treatments due to differences in organ filling or weight gain and loss. IGRT uses images, such as CT, stereoscopic X-rays, are obtained with patients in the exact position they will be during treatment and compares them every day to similar images taken in the treatment room just before the patient is given the radiation treatment. Our equipment lets our doctors fuse the two sets of images every day to see if the treatment beams need to be adjusted, typically by one to a few millimeters. This allows your doctors to better target the cancer while avoiding nearby healthy tissue. In some cases, when there are no reliable landmarks for IGRT, your doctors will implant a tiny piece of material called a fiducial marker near or in the tumor to help them localize the tumor every day during IGRT.