Intensity modulated radiation therapy (IMRT) is a powerful technology that can be used to treat many different tumors, including cancer of the prostate, breast, brain, lung, head and neck. The list continues to grow.
In IMRT, very small beams, or beamlets, with varying intensities can be aimed at a tumor from many angles. The intensity of each beamlet can be controlled. During treatment, the beam shapes change hundreds of times. The radiation dose can be made to bend around important normal tissues in a way that is impossible with other techniques.
Because these motions are so complicated, special high-speed computers, treatment-planning software, multileaf collimators, diagnostic imaging and patient-positioning devices are used to plan treatments and control the radiation during therapy.
IMRT may improve the effectiveness of radiation therapy by delivering more radiation dose to tumor cells while reducing the exposure of surrounding cells. A special computer planning system and complex, precise mathematical computations are used to calculate the dose and distribute the radiation. IMRT uses a technique known as inverse treatment planning. The desired outcome is entered into the computer, and the computer determines the ideal beam arrangement and intensity. The computer can craft a treatment plan from a range of possibilities greater than would be practical for a clinician to test one by one.
For IMRT to be effective, the anatomical position of the tumor and surrounding normal tissues must be accurately defined. CT (computed tomography) and MR (magnetic resonance) imaging provide the necessary three-dimensional anatomical information. It is also important to accurately position and immobilize the patient during treatment. IMRT is usually given five days a week for six or seven weeks. The total dose of radiation and the number of treatments needed depend on the size, location and kind of cancer, the patient’s general health and other medical treatments the patient is receiving.
An advantage of radiation therapy is that it affects only the tumor and surrounding area. The challenge is to improve the planning and delivery of radiation beams to effectively target the tumor while reducing the damage to normal tissue and subsequent side effects. Three-dimensional conformal radiation therapy (3-D CRT) combines digital diagnostic imaging, powerful computers and specialized software to conform the radiation beam to the shape of the tumor.
Not only does IMRT use three-dimensional imaging and treatment delivery, it also allows varying intensities of radiation to produce dose distributions that are far more “conformal,” or precise, than those possible with standard 3-D CRT.
IMRT does not completely eliminate the radiation dose to surrounding tissue, but it reduces it substantially. This decreases side effects and makes it possible to deliver a higher radiation dose to the tumor, which increases the probability the tumor can be completely destroyed.
IMRT finally makes radiation therapy an option for treating types of cancer that may not have been otherwise treated, giving patients new hope for cure