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.
While sometimes utilized within the brain, IMRT’s most promising use will be treating body cancers with less damage to healthy tissues. More precise techniques using one-session Gamma Knife® machines and other one-session linac technology are best utilized within the brain. Several manufacturers currently offer beneficial treatments with high-level linac technology that can perform both one-session radiosurgery and radiotherapy. The most well-recognized brand names at this time are the Novalis Tx® (Varian Medical Systems, Inc. and BrainLab, Inc.), Synergy S® (Elekta, Inc.), CyberKnife® (Accuray, Inc.). All of these machines are robotic and image-guided and can perform IMRT.
Patients should be aware of marketing efforts by manufacturers and hospitals that imply one linac technology is an improvement over another, either by the way it delivers radiation or by the way it images tissues. The biological effect and probable outcome will be the same regardless of whether a gantry or robot is used for delivery. Conformal radiation treatments have the ability to deliver a higher radiation dose within the tumor, thus causing more damage to the tumor and less damage to surrounding healthy tissues than conventional external beam radiation treatments. One manufacturer of conformal radiation treatments states that the radiation falls within four millimeters of the predicted positions. This is a great improvement over conventional radiation therapy, but not as accurate as one-session intracranial radiosurgery where a precision of one-third to one millimeter can be obtained.
IMRT enables a more precise conformal radiation dose distribution to the target area by allowing the physician to control the intensity of the radiation beam within a given area. Again, this means a much higher dose of radiation may be given to a tumor without an increase in radiation delivered to the normal tissue. IMRT utilizes beams or multileaf collimators that can turn on or off or be blocked during treatment, varying the radiation beam intensity across the targeted field.
The radiation beams may be moved dozens or hundreds of times and each may have a different intensity, resulting in radiation sculpted in three dimensions. The healthy surrounding tissue receives a smaller dose of radiation than the tumor does. Thus there is no longer a homogeneous or even radiation dose, but a dosage that can be made higher or vary within the tumor, similar to what the cobalt-60 Gamma Knife® accomplishes with one-session stereotactic radiosurgery within the brain. (However, the precision is much greater with Gamma Knife® surgery than with conformal radiation.) The end result is better tumor control, less damage to healthy tissues and structures in the treatment area and a better quality of life for the patient.
Treatment planning for IMRT and other conformal radiation is more complex than for conventional radiation therapy, taking an average of 2–3 days for each patient. Three-dimensional planning for conformal radiation versus simple one-slice planning for conventional radiation therapy extends treatment planning time for the patient.
This means that a patient will typically be required to come in for several scans. Additionally, the patient will be fitted with a reusable localization device, which may be a mask, body frame or other device. These devices assist the radiation delivery machines in targeting with more accuracy. Frequently, the localization device is molded to fit the precise contours of the individual patient. Alternatively, a body frame may be used. The molded device or body frame will be placed on the patient each time he receives a treatment. Multiple treatments are usually required with conformal radiation, the same as with conventional radiation therapy. The number of treatments may range from 1–28, which is less than with most conventional radiation treatments. Actual treatment time for each session is typically longer than with conventional radiation therapy because of the complexity of the treatment itself.
Radiation treatments given daily or multiple times a day are called fractionated treatments. As with conventional radiation treatments, by giving the treatments in a fractionated manner, the normal healthy tissue that does receive minor radiation overlap is thought to be able to regenerate itself faster than the tumor can, and therefore be less damaged. Additionally, radiation given in fractions is able to reach the tumor cells during different stages of cell growth, possibly causing more damage.
Stereotactic radiosurgery machines (like the Gamma Knife® and high-level linac technology) are able to complete treatment in one session due to their precision and ability to immobilize the target area within the brain better than we can immobilize targets within the body.
If a brain tumor patient is not a candidate for one-session stereotactic radiosurgery, he may be a candidate for one- or multi-session radiation therapy treatments utilizing high-level linac radiation machines. Conformal radiation and IMRT are a significant step forward in treatment and control of body cancers, with less damage and side effects. Although a relatively new innovation, there exist clinically relevant data on conformal radiation with several body cancers, including prostate, lung and liver cancers.
The side effects of IMRT are the same as those of conventional radiation therapy (see Radiotherapy Overview and Radiation Therapy) but tend to occur less frequently and with less intensity in the short and long term. IMRT and conformal radiation therapy are deliverable anywhere within the body, as long as the affected area is properly immobilized. As with conventional radiation therapy, there is no limit to the size of tumor that can be treated. Patients who have previously received the maximum amount of radiation deliverable by conventional radiation therapy are able to be treated with IMRT and other conformal radiation therapy.
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.
As previously stated, before the advent of conformal radiation therapy the maximum radiation dosage was restricted by the impact it would have on nearby tissues. For instance, when conventional radiation therapy is utilized to treat lung or breast cancer, some overlap of radiation may occur to the arteries of the heart. In some instances, this can cause these arteries to thicken over time, restricting blood flow. This may or may not be a problem, depending on the blood flow within the other heart arteries and the patient's overall health. For a few patients, this may necessitate a bypass operation in the future. With conformal radiation therapy and IMRT for the lung or breast tumor, the radiation overlap to the heart arteries is shown to be minimal or nonexistent, possibly eliminating the need for further invasive care and/or heart problems.
Future studies will further confirm or deny the potential positives of treatment with IMRT and other conformal radiation.