In order to understand what intensity-modulation is, and why it is important, you’ll need to understand some basics about conventional radiation therapy and how it is used.

The radiation beam produced by essentially all modern radiation therapy machines is similar to the beam produced by the regular “diagnostic” x-ray machine with which you are already familiar. The x-rays are produced in the machine electrically; there is nothing radioactive inside. The beam of radiation comes out as a rectangular “cone” – just as in the light beam of a slide projector or movie projector. In a diagnostic machine, the x-rays that come out the other side of the body hit a film or detector, and produce an image of what is within the body. In a therapy machine, the beam passes through the body and is absorbed into the thick walls and floor of the treatment room. On the way through the body, however, the x-rays kill cancer cells in the target area. They also, unfortunately, kill some good cells on the way to the target and on the way out the other side.

[The question of how the radiation kills more cancer cells than normal cells is a good one, and well beyond this discussion of radiation technology. In a very simple nutshell, radiation is more likely to kill a cell if it is undergoing cell reproduction. Since cancer cells usually reproduce more often than normal cells, they are more vulnerable to being killed by any single treatment. After a series of treatments, the radiation will have killed a far higher proportion of cancer cells than of normal cells. When it is possible to kill ALL of the cancer cells, and only SOME of the normal cells such that the body can repair itself, we call it “CURED”!]

There are two main differences between a diagnostic x-ray machine and a therapy machine (linear accelerator, or “linac”).

The therapy beam has an x-ray energy that is up to 100 times higher than the diagnostic beam. This means that far more of the beam penetrates deeply and through the body, and it is not stopped by bones or other dense materials that would stop a diagnostic beam.

The therapy beam produces far more x-rays than are needed to take make an image. The amount of radiation energy deposited in the body by a single treatment is hundreds, or even thousands, of times more than a single chest x-ray.

A simple way to think of the difference is to compare a squirt gun to a fire hose. The fire hose delivers much more water, at much higher pressure.

Earlier we compared the “cone” of a radiation beam to a slide projector. There are several way in which the two are similar.

In both cases the beam, once it leaves the source, will travel in a straight line until it is absorbed; it cannot be bent or steered in any way.

The shape and size of the cone can easily be altered by placing shutters or absorbing blocks within or on the edges of the beam.

Both the slide projector and the x-ray machine are carefully optimized to produce a beam that is even, or “flat”, all the way from corner to corner.

And in both cases, the intensity of the beam can be modified if a filter or absorber is placed in the path of the beam. This absorber can be a simple “flat” filter that reduces the intensity of the beam uniformly, resulting in a beam that is still flat, but less powerful. Or it can be a more complex filter that reduces the intensity of some parts of the beam more than others, resulting in a beam that is no longer flat. This is intensity modulation.

In real life, the body and the target are three-dimensional. The shape of the body, the position and shape of the target, and the position and shape of the other normal structures will change from slice to slice. The calculations needed are far beyond the ability of a human, and did not become feasible until computers were able to make very complicated, three-dimensional calculations. Only a few years ago, the University of North Carolina arranged to borrow time on a Cray Supercomputer to explore IMRT. Now it is done by sophisticated computers in all major radiation therapy centers, and many smaller centers as well.

The actual modulation of the radiation beam can be done in a number of ways. Some places actually construct or cast custom brass filters for each patient, one for each radiation beam employed. These are placed into the beam at the head of the linear accelerator and they attenuate the beam just as described in the example above. Most centers use a device called a multi-leaf collimator at the head of the accelerator. It operates under computer control to give many short bursts of radiation from each of the beam directions, while changing the shape of the beam between bursts. Areas covered by many bursts will get more radiation than areas that are covered by just one burst, or only a few. Two such machines, at our centers, are the Siemens Primus and the Varian Trilogy. Another of our treatment units, TomoTherapy, was designed specifically to do IMRT. It produces 64 small “beamlets”, each of which can be turned on or off rapidly and independently, and the entire array of beamlets rotates 360 degrees around the patient during the treatment. The capabilities of the machines are similar, but not identical, and your doctor will be able to explain more about your particular case and why a particular unit was chosen.

IMRT is not just a matter of buying a machine and putting it to work. It involves both computer equipment, for planning the treatment, and large multi-million-dollar linear accelerators for administering the treatment. Medical Physicists assist the radiation oncology physicians in overseeing the whole process, and are responsible for the large amount of ongoing quality-control that is necessary. Dosimetrists take the dose prescription of the physician and operate the computers that turn it into an optimized treatment plan for the doctor’s approval. Radiation therapists care for the patient directly, assuring that positioning is correct and reproduced accurately, and then operating the complex, computerized process of actually administering the treatment.

IMRT also places real demands on the radiation oncologist. Advances in medical imaging have made it more possible than ever before to know the position, size, and shape of the tumor, and the adjacent structures where radiation dose should be minimized. The radiation oncologist must, however, also use his or her knowledge and oncologic judgment to decide what areas of the body might harbor undetectable cancer than should also be treated, and to know what doses of radiation are necessary for cure, and tolerable for normal tissues. With the enormous flexibility of IMRT, it is possible to err becoming too aggressive and causing damage to normal tissues, or by being too conservative and failing to give an adequate dose to all the cancer cells. This means that the radiation oncologist must be not only well-trained, but current in all the latest knowledge and applications of IMRT.

In recent years, there has been a good deal of advertising of cancer services that would lead one to believe that a certain treatment machine is better than all the rest. This is simply not true. A wise patient first selects the radiation oncologist, and the radiation oncology team, that he or she trusts most to deliver quality care. Choice of the specific treatment tools and methods can then be left to the judgment of the radiation oncologist. At Radiation Oncology Associates, we offer almost every current technology in radiation therapy. If a patient should require a treatment we cannot provide, he or she will need to leave the Baltimore/Washington area to obtain it. Our radiation oncologists usually have personal relationships with the doctors at these specialized facilities, and do not hesitate to telephone and facilitate a referral when appropriate.