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Intensity Modulated Radiation Therapy (IMRT) is one of the newest and most advanced ways of giving precise doses of radiation from outside the human body with a linear accelerator.Traditionally, radiation has been delivered through a limited number of positions from outside the body, using a rectangular beam of radiation. This rectangular beam is then shaped with thick blocks to contour the radiation beam as desired. Where these 2 to 6 beams overlap, there is a uniform dose of radiation. This uniform radiation cloud is designed to be placed around the organ containing the cancer, so that the tumor and the surrounding normal tissues all get the same dose of radiation.

IMRT uses a different approach. Instead of a uniform “dose cloud” of radiation throughout the treated area, each point in the treated area can be treated to a different dose of radiation (that is the intensity of the radiation is modulated, or changed.) To do this, generally 5 to 9 main beams of radiation are used, each aimed at the treated area from different angles. But instead of each beam having the same dose throughout, each beam is broken up into tiny squares of radiation (“pixels”). This is much like High Definition Television, making the same size picture out of many, many tiny squares. Each of these ‘pixels” makes a “beamlet” (small beam) of radiation; thus, each of the 5- 9 main beams of radiation may be broken up into 80 or more “beamlets”. In effect, then, hundreds – or even more than 1000!- little beams of radiation are aimed at the tumor from different angles. This allows us to give very high doses of radiation to the tumor, and give far, far lower doses to a normal structure right beside the cancer area. Instead of the entire area getting the same dose everywhere, the radiation dose is purposefully varied throughout the treatment area in a designed fashion. Obviously, this process is very complicated. It became possible as computers became more powerful, and depends on a planning process called “inverse planning.”

As noted above, traditional radiation used shaped “blocks” in the radiation beam to contour the radiation beam into a desired shape. IMRT was started using MLC’s (Multi Leaf Collimators.) MLC’s are thin metal bars that are moved by miniature motors. Each metal bar can be moved into or out of the radiation beam by the motors. By controlling how far into the beam each MLC moves, the radiation beam can be shaped without using fixed blocks. The MLC’s can be used to create the “beamlets” of radiation for each beam. One method is to move the MLC’s into one desired shape, turn the beam on for the determined amount of radiation, then turn the beam back off. The MLC’s are then re-shaped into the next desired shape for that beam, and the process is repeated. By doing this with multiple different designed MLC patterns, the desired “beamlet” pattern is created. This method is called “step and shoot” IMRT. Another method is to move the MLC’s gradually through the beam, each at different rates, while the beam is turned on. This method is called “dynamic leaf‘ IMRT.

After IMRT became possible with MLC’s and Inverse Planning computer systems, the next step in the development process was Compensator based IMRT. This involves the same planning and concept process as above. However, instead of shaping the radiation “beam lets” with MLC’s, a solid piece of brass is designed for each planned radiation main beam. The milled brass device is of differing thickness, for each tiny “pixel”- thus, when it is placed in the radiation beam, each different thickness “pixel” creates the desired “beamlet” of radiation. Compensator- based IMRT has several potential advantages over MLC -based IMRT: the treatment times are usually much shorter for the patient; there is no undesired ‘leakage’ of radiation as there is between the MLC “leaves”; there are no motors or moving parts to stick and allow unplanned doses of radiation to be given, as can happen not infrequently with MLC motors.

Central Indiana Cancer Centers has IMRT available at all of its centers. All sites can use compensator-based IMRT. The East Cancer Center has a linear accelerator with 120 miniature multi-leaf collimators to shape the “beamlets” of radiation as an alternative as well. (MLC’s are used to shape radiation fields at our other centers.)

It takes careful and in-depth planning and physics verification to accurately deliver IMRT treatments. ClCC has a highly-qualified physics and therapist team, and uses the newest Inverse-Planning treatment computer from CMS to help the physicians and physics team preplan the IMRT treatment delivery. Careful Quality Assurance is done before and throughout the treatment process to assure that treatment is delivered in the best possible way.

“IMRT Compensators”