How We Hit a Moving Target in Radiation Therapy
By Mike Taylor
Radiation is a treatment that we can focus precisely to kill tumors. We extensively evaluate a tumor’s location and surrounding tissues to ensure that we target tumors accurately and minimize side effects. We also must evaluate the movement of a tumor, to make sure the entire tumor is treated. Many tumors do not move, such as those in the brain, while other tumors, such as prostate cancers, move with changes in the fullness of the bladder and rectum. Before treatment delivery, we use X-rays or CT scans to confirm that the radiation is targeting exactly where your doctor intends, through a process called image guidance.
Some tumors, such as those in the lung, present a unique challenge as they can move much more during treatment delivery. Therefore, we take a special planning CT scan (4-dimensional) that acquires multiple CT images while breathing normally. This essentially creates a video of the tumor’s motion while breathing. This 4D CT scan allows us to accurately visualize the tumor movement and target it.
If your physician believes your tumor has excessive movement or is close to normal tissue, they may recommend respiratory gating. This technique involves treating the tumor when it is in a specific location or is moving very little. This technology accounts for tumor movement and can limit the nearby normal tissue irradiated. Your physician will choose if this technique is the best treatment for you during treatment planning. At that time, your doctor and a team of physics specialists will choose a portion of the breathing cycle with the least amount of tumor motion.
If your doctor recommends respiratory gating, during your radiation treatment a marker block is placed on your chest. As your chest rises and falls with breathing, the marker block moves. This movement can be used to track tumor movement in the lungs. Then the treatment machine can be configured only to deliver radiation when the tumor is in a particular position.
Your body positioning and tumor movement are carefully evaluated using CT scans just before treatment delivery. The physician, radiation therapists, and physicists carefully review the images and ensure that the tumor motion and position match your doctor’s plan. Once the position is verified, daily treatment delivery is completed within minutes.
Respiratory Gating is one of the many advanced techniques and technologies that Alliance Cancer Care uses to treat tumors with greater accuracy while minimizing side effects.