External Beam Radiotherapy for Prostate Cancer
How is IMRT Done?
There are two new concepts that define contemporary IMRT for prostate cancer. 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 (FIGURE 12). 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 (FIGURE 13).
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 (3).
A final note is that IMRT is only as good as the imaging that guides it. Because of this, Radiation Medical Group had also invested 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 (FIGURE 14). 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 (FIGURE 15) allows the most precise sculpting of the radiation beam, to fit the exact limits of the prostate 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, RMG has also invested in the BAT system (FIGURE 16), 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.
RMG is currently the largest IMRT provider in the San Diego region. We continue to intensively research and develop the IMRT technique in our practice, and will always work to the absolute best of our collective ability, to remain at the forefront of this exciting new technological prostate cancer treatment breakthrough.
