IMRT is short for Intensity Modulated Radiation Therapy. The intensity of the radiation in IMRT can be changed during treatment to spare more adjoining normal tissue than is spared during conventional radiation therapy. Because of this an increased dose of radiation can be delivered to the tumor using IMRT. Intensity modulated radiation therapy is a type of conformal radiation, which shapes radiation beams to closely approximate the shape of the tumor.
Local or regional control of a tumor is the ultimate goal of an overall treatment strategy, especially for a patient with cancer. Failure to achieve tumor control can result in a greater likelihood of developing distant metastases, continued tumor growth, severe debilitation or even death of the patient.
Historically, the maximum radiation dose that could be given to a tumor site has been restricted by the tolerance and sensitivity of the surrounding nearby healthy tissues. When a tumor or condition is not eligible for treatment with normal stereotactic radiosurgery, conformal radiation may be used in one or more sessions. Three-dimensional conformal radiation therapy is less than ten years old. It is only available with linear accelerator-based technology. In 2006, there are finally enough linac centers offering image-guided three-dimensional conformal radiation therapy that we soon expect to see more research articles with relevant 5-year followups, upon which patients can rely.
How It Works ?
A more standard radiation therapy called three-dimensional conformal radiation therapy (3-D CRT) uses digital diagnostic imaging, computers and special software to conform the radiation beam to the shape of the tumor. IMRT is the latest advance in 3-D CRT technology. IMRT involves the use of varying intensities of hundreds of small radiation beams to produce dosage distributions that are more precise (conformal) than those possible with 3-D CRT.
In intensity modulated radiation therapy (IMRT), very small beams, or beamlets, are aimed at a tumor from many angles. During treatment, the radiation intensity of each beamlet is controlled, and the beam shape changes hundreds of times during each treatment. As a result, the radiation dose bends around important healthy tissues in a way that is impossible with other techniques. Because of the complexity of these motions, physicians use special high-speed computers, treatment-planning software, diagnostic imaging and patient-positioning devices to plan treatments and control the radiation dose during therapy.
For IMRT to be effective, the anatomical position of the tumor and surrounding healthy tissues must be accurately defined. Computed tomography (CT), positron emission tomography (PET) and magnetic resonance (MR) imaging provide the necessary three-dimensional anatomical information. It’s also important to accurately position and immobilize the patient during treatment.
This may be done with special head frames (if the head or brain is being treated), or with advanced imaging devices such as electronic portal imaging and scanning ultrasound, which provide daily information about the location of internal organs. Some organs, such as the prostate, move due to normal daily volume changes in the bladder and rectum. Gold seeds may be placed into the prostate to track prostate movement daily and ensure more precise targeting.
A device called a multileaf collimator adjusts the size and shape of the computer-determined radiation beams. The collimator, a computer-controlled mechanical device, consists of up to 120 individually adjusted metal leaves. These leaves move across the irradiated tissue while the beam is on, blocking out some areas and filtering others to vary the beam intensity and precisely distribute the radiation dosage.
Radiation oncologists usually administer a regimen of IMRT treatments over four to eight weeks. The total dose of radiation and the number of treatments given depend on the size, location and type of cancer; the patient’s general health; and other medical therapy the patient is receiving.
The areas most commonly treated with IMRT are: prostate, spine, lung, breast, kidney, pancreas, liver, larynx, tongue and sinus. The brain is treated with IMRT when one-session radiosurgery is not appropriate or unavailable.
As with conventional radiation therapy, treatment with IMRT or other conformal radiation always involves a radiation oncologist and physicist. Should the treatment site be within the brain, a neurosurgeon is required to be a part of the team and the patient should insist upon it.