By Dr. James F. McNab
Beaufort Memorial Hospital
Much of the recent progress we have enjoyed in the field of radiation oncology has coincided with the technological revolution in diagnostic imaging. While we now take CT scans, MRI scans and PET scans for granted, it was only a few short years ago that conventional X-ray pictures of the chest, abdomen or pelvis were the primary imaging procedures.
One can’t easily treat what one can’t see, and surgical explorations were formerly much more common, simply because we could not well visualize the normal anatomy and did not trust ourselves to make major clinical decisions on the basis of fuzzy little shadows.
Only a few years ago, the standard radiation treatment field for prostate cancer was a 6-inch square, aimed in the general direction of the pubic bone, which served as an anatomic reference point. The inability to accurately localize the gland resulted in excessive radiation exposure to the bladder and rectum.
Although early clinical trials proved effective in controlling the cancer, patients paid a tremendous price in terms of normal tissue complications. The same can be said for patients with breast cancer, who formerly experienced an unacceptably high incidence of cardiac and lung injuries, as a result of their treatment.
As a radiation oncologist, it is always our desire to maximize cures rates and minimize the risk of complications. It’s all about treating with tight margins and knowing where the target is located at any point in time.
Both tumors and internal organs naturally move, and that is why radiation treatment planning and tumor localization, performed only at the outset of a six-week course of treatment, may not be valid further along in the course of therapy. When organs move during radiation treatments, the tumor may not get the right amount of radiation and other nearby organs may receive treatment that is not meant for them. To insure optimal dose delivery, daily adjustments in the treatment volume are required to accurately track changes in the anatomic position of the tumor and its relationship to normal adjacent structures.
Thanks to our affiliation with Duke University, we are fortunate to have the most advanced radiation treatment imaging technology, including the latest Varian Clinac Linear Accelerator with IMRT/IGRT capabilities; On-Board Imaging; Respiratory Gating; and Image Fusing. I personally review each and every treatment field to ensure proper tumor targeting and dose delivery.
On-Board Imaging allows us to see the position of the target immediately prior to each treatment. Essentially a CT scanner built directly into the treatment machine, the system facilitates precise tracking of the target and permits real-time adjustments in the treatment volume, in accordance with daily variations in organ position.
In the case of prostate cancer, we literally follow the bouncing ball. As the gland moves, due to changes in the amount of urine in the bladder or gas in the rectum, we can follow right along. As a result, our treatment margins are now only a few millimeters and the collateral damage to the adjacent organs becomes negligible. I find that our younger therapists, who have grown up zapping space aliens with video games, are particularly adept at operating the new equipment.
Respiratory Gating is a process by which the radiation treatment is delivered in multiple brief pulses, rather than a continuous stream. Particularly useful in women with breast cancer, the technology permits us to visually track the position of the breast and sequence the radiation delivery, in accordance with the natural movement of the chest during inhalation and exhalation. In contrast to earlier techniques, which resulted in unnecessary heart and lung exposure as these organs moved into and out of the path of the beam, we can now focus on the target and largely avoid damage to adjacent normal tissues.
Image Fusing allows us to simultaneously overlay multiple images obtained from different sources and take maximum advantage of each imaging source. (For example: CT scans to see bones, MRI scans to see soft tissues, and PET scans to assess metabolic activity.) With this computer-enhanced digital technology we see one crystal clear picture, which can be easily magnified. Millimeters matter at the Beaufort Memorial Keyserling Cancer Center in Port Royal. We take our responsibilities seriously, and continuously strive to improve the accuracy and precision of the care provided.
Learn more at www.bmhsc.org/cancer.
James F. McNab, MD, is a board-certified radiation oncologist at Beaufort Memorial Hospital – Keyserling Cancer Center.
