Intensity Modulated Radiation Therapy (IMRT)
by Donald Fuller, MD Radiation Medical Group, Inc
This new powerful computer-driven method of radiation for prostate cancer, IMRT, followed soon after the adoption of three-dimensional conformal radiotherapy and is the new standard external radiation treatment method by American centers of excellence.
Simply stated, IMRT is three-dimensional conformal radiotherapy, but represents a much more sophisticated and robust method of three-dimensional radiation planning and delivery. With standard three-dimensional conformal radiation therapy (3DCRT), each beam typically has a uniform intensity across its entire area. This is not the case with IMRT.
Intensity Modulated Radiation Therapy (IMRT) allows a much greater degree of control of the radiation volume, by varying the strength of a beam across its entire area. This is known as modulating the intensity of the beam while the radiation is on. Effectively, this converts one large beam into many tiny “beamlets,” each with its own customized intensity, designed by the computer. These beamlets may assume complex patterns effectively allowing control of a new radiation physics dimension in the planning process. By cross firing through the tumor volume with a number of these intensity modulated beams, the physician may deliver a much more highly conformal volume of radiation to a target volume, while simultaneously better protecting protect sensitive, adjacent organs.
The superiority if IMRT becomes particularly prominent when the target volume has a complicated shape or curves around nearby normal organs. The IMRT method allows the high-dose radiation volume to be effectively curved to fit virtually any volume requirement. In the case of prostate cancer, the prostate target volume usually curves to some degree around the rectum, and thus typically represents an area in need of improved radiation dose distribution. In response to this need, the IMRT high-dose radiation volume may be made to conform to the prostate +/- seminal vesicles plus a specified margin, while curving inward where the rectum encroaches (figure 10), to better protect it. This is a major advance compared with standard three-dimensional conformal radiation therapy, though there are still many situations where standard 3DCRT is used.
How is IMRT Done?
There are two new concepts that define contemporary IMRT. As discussed above, the first is the computer-designed modulation of the intensity across a radiation beam during treatment. This is accomplished by dividing the treatment beam into many pencil-sized beamlets. Each beamlet arrangement is then computer-programmed to hit areas within the target for a specific amount of time, and then move in rapid succession to the next beamlet arrangement, until the entire coverage objective has been met. This process is repeated from multiple different angles
Computer programmable Multileaf Linear Accelerators (figure 11) allow this to occur. Radiation Medical Group had made a very large capital investment in these treatment devices in anticipation of IMRT. These devices produce the high-energy therapeutic radiation (X-Ray) beam and contain 80 or more individual thick lead alloy leaves, driven by 80 or more small computer driven motors, allowing the creation virtually an infinite possibility of beamlet arrangements by specifically blocking computer prescribed aspects of the larger beam.
The second IMRT concept is known as inverse planning. This is a major departure from conventional radiation therapy, where forward treatment planning is utilized. Forward planning means the radiation oncologist and physicist design the beams, and the computer proceeds according to their instruction. They then review the computer-generated dose to the tumor and surrounding tissues based on what has been programmed. If they are not satisfied with the radiation dose distribution, they will then need to redefine alternative doses, beam angles, and/or blocking designs until a satisfactory plan is produced. This iterative process may be repeated up to 3-4 times per case. For standard situations this method works well, but for complicated situations it is inefficient and very limiting of the final planning product.
With inverse planning, the trial and error method characteristic of forward planning is replaced by a method wherein the radiation oncologist and physicist prescribe target volume dose coverage “objectives” and normal tissue protection “objectives.” The computer will then set about to creating a custom intensity modulation plan until it satisfies the prescribed objectives, typically running through many dozens or hundreds of “iterations,” until an ideal volume coverage result is created.
The final IMRT product is a highly customized radiation therapy volume that may tightly wrap around virtually any tumor volume configuration. In properly selected patients, this will translate to a higher probability of cancer eradication and/or a lower probability of radiation therapy complications. The Memorial Sloan Kettering Cancer Center report provides compelling evidence that this translates to a major improvement in disease-free survival for prostate cancer patients, even as it further reduces the normal tissue complication risk compared with standard three-dimensional conformal radiation therapy.
A final note is that IMRT is only as good as the imaging that guides it. Because of this, ask your doctors about the extent your hospitals investment in capital and personnel dedicated to creating the ability to use both prostate CT and prostate MRI images in the planning process, to more accurately define the target volume . The CT is necessary to define all of the structures and body contours in the entire irradiated volume, while the MRI gives a much finer definition and image of the cancer itself, within and around the prostate. Both image sets are registered in the computer and fused one on top of the other, to take optimal advantage of both image sets in designing the final radiotherapy volume product. This image fusion process allows the most precise sculpting of the radiation beam, to fit the exact limits of the cancer volume.
Even the most sophisticated beam design arrangement may still potentially miss the tumor volume if the prostate is moved by the bladder or rectum, which can and does happen. To negate this possibility, ask your doctor if your hospital has also invested in the BAT system, an ultrasound-based computer device that measures the patient’s prostate location daily, and then prescribes corresponding millimeter adjustments in the beam alignment every day, to follow any prostate movements. This will cause the beam centers to exactly track the prostate center daily, always keeping the prostate target volume squarely within the high-dose radiation volume.
information last updated on May, 2005