By Richard Pearlman, M.D., radiation oncologist at Alliance Cancer Care
At Alliance Cancer Care, we use daily image guidance for many of our treatments. If you are receiving treatment with us, you are probably undergoing some type of imaging regularly. Image-guided radiation therapy, or IGRT, is a method that uses in-room x-ray or CT to visualize the area that is to be treated. You may hear us refer to this as “daily alignment,” “setup imaging,” “daily films,” and probably many other variations. What all of these phrases have in common is that some type of imaging is performed to ensure that the treatment delivered is accurate.
How are the images obtained and used?
Before a treatment, we verify the setup accuracy by performing a cone beam CT, MV, or kv x-ray images. We use these images, taken at the time of your treatment, to ensure that the target area matches that of the original simulation CT scan from which your treatments were planned. If the two images match, then we know that the treatment delivered will be accurate. If there is any mismatch detected in the setup image, the radiation therapists make small corrections in your position by moving the robotic treatment table, which moves your body into the exact position before treatment starts. This setup process is most often performed daily, but sometimes, we will do it less frequently, such as every five treatments.
What are the types of images used?
The most detailed type of image is the cone beam CT. A cone beam CT is very similar to a diagnostic CT, but due to the way it is acquired, it does not have the same image quality as a diagnostic scan. It also does not use diagnostic protocols or contrast, so its ability to see the tumor being treated is often limited. Therefore, we usually do not make judgments regarding your tumor’s response to treatment using this type of imaging. However, we can still use your cone beam CT to check for alignment of soft tissues, such as fat, muscle, and internal organs. We can also check for bladder filling and stool or gas in the bowels. Patients being treated for prostate cancer are the most familiar with this process since we check a daily cone beam CT to properly align the prostate before treatment, and we also evaluate the bladder to make sure that it is full and that the rectum is empty.
Regarding the two different types of x-rays that can be performed on the treatment machine, there are MV, or megavoltage, and kv, or kilovoltage, images. These terms reference the energy of the x-rays used to perform each image. The MV beam uses the same beam for imaging as treatment. It may have slightly less detail than a kv image due to the higher energy. This is a complicated physics phenomenon of how x-rays of different energies interact with different tissues. But since the MV image uses the treatment beam, a beam’s eye view of the patient’s positioning can be evaluated. Kv imaging is especially useful when setting up accurately to the bone. Both kv and MV imaging can be used to take images at 90 degrees to each other, which can be used to align the target area in 3 dimensions vs. 2 dimensions with just one view. We decide on which of these types of imaging to perform based on the individual clinical scenario.
In contrast to a cone beam CT, which shows both bone and soft tissue, MV and kv images show bone more than soft tissue. You might wonder, “Why don’t we use a cone beam CT for all of our treatments?” This is a good question. Firstly, not all treatments require the level of soft tissue detail provided by a CT. If we are treating cancer in a bone, it is not necessary to have the soft tissue detail. Secondly, a CT takes more time to acquire. Sometimes, lying on the treatment table can be uncomfortable, especially for people with back or joint pain, or the patient being treated may be very sick, such as an ICU patient. At times, reducing a patient’s time on the treatment table is a priority.
What is gated imaging?
In addition to the various types of imaging that we described, we can also choose to perform imaging using the fourth dimension of time. For example, when we treat patients with lung cancer, we often use what we call 4D- or gated imaging, which accounts for the factor of time. Before treatment, we can program the machine to image the patient at certain times, such as during inspiration, expiration, or any time in between. Using this type of image guidance, we can account for tumor motion and often treat smaller areas of normal tissues. We can perform this type of imaging by tracking your breathing during the initial planning scan and in the treatment room. Please watch Dr. Everett’s video for more information.
What is surface guidance?
We also use surface guidance for accurate setup and for detecting motion in our patients before and during treatment. This process uses an infrared camera system, which is part of the Varian Identify® system. Learn more about the Identify® system here.
Why does image-guided radiation therapy matter?
Image-guided radiation therapy is a technique and technology we use to accurately deliver your treatments regularly. Various forms of imaging can be performed, such as cone beam CT, MV images, and kv images, and your doctor will select the best option for you based on individual factors about your tumor and your treatment. Each method has its pros and cons. In addition to these imaging techniques, we can employ gated imaging, which allows us to align the target area using breath-hold techniques or during specific times while breathing. We use these various techniques when indicated to help us deliver the most accurate treatments possible for our patients.
If you or a loved one has been diagnosed with cancer and would like to learn more from our team of experts, please contact our office to learn more about consultation.