How We Use Secondary Imaging in Radiation Oncology
By Bart Lynch, Medical Physicist
If it is recommended that you undergo radiation therapy, one of your next appointments is for CT simulation. Most patients receive a computed tomography (CT) scan, or treatment planning CT. The images from the CT scan are then used for target identification, treatment planning, and dose calculation. Depending on your specific type of cancer, your physician might request that other imaging, such as an MRI or PET-CT, be performed. These secondary imaging modalities can provide additional information about the tumor’s position, allowing your physician to target the tumor better and spare normal tissue. Dr. Everett describes the types of scans for cancer patients here.
PET, which stands for positron emission tomography, is a functional imaging technique. Functional imaging focuses on detecting changes in biological processes, such as metabolism, blood flow, or absorption. For a PET scan, a radioactive element is attached to a drug, hormone, or chemical, and called a radiotracer. This is injected into the body and a specified time elapses before a scan is done. During this time, the injected radiotracer migrates to the site of interest and emitted gamma rays are detected to create an image. A diagnostic CT scan is often acquired simultaneously to improve the ability to detect the anatomy of where the PET uptake is found.
The PET radiotracer is chosen based on the process within the body that your physician would like to image. For cancer patients, the radiotracer most often used is F-18 FDG. Radioactive fluorine (F-18) is attached to a glucose sugar (FDG) absorbed by tissues actively using glucose. For example, the brain, heart, and liver are typically “brighter” on a PET because they use sugar as their primary nutrient. The kidney and bladder also are “bright” on a PET, because the dye is filtered through the kidney and expelled through the urine.
However, F-18 FDG isn’t the only radiotracer available. A recently FDA-approved radiotracer allows the imaging of prostate cancer cells. Gallium-68 is used to target prostate-specific membrane antigen (PSMA), a protein in prostate cancer cells. It is used in patients with suspected metastasis before initial treatment and with suspected recurrence based on increased prostate-specific antigen (PSA) levels.
MRI stands for magnetic resonance imaging and uses magnets to produce images instead of X-rays. MRI scans are performed on a certain area of the body, and are not “whole body.” MRI scans produce excellent soft-tissue images and help look at the brain, liver, breast, pelvis, or muscles. MRI is often used in conjunction with other imaging types. For example, since PET scans light up the entire brain, MRI is essential to see the fine detail of the brain and to detect the presence of a tumor there.
MRI uses a strong magnetic field, which causes vibration of the water molecules in the body. As the magnet turns on and off, the machine measures the “relaxing” of the water molecules back into their normal state. This produces images that represent soft tissues inside the body. MRI scans are typically longer scans, and the machinery is very loud. Be sure to alert your doctor if you have any metal in your body, like a pacemaker or defibrillator, before your MRI scan.
How are these images used?
Your doctor may request a PET or MRI to help with defining the area to treat for your cancer. Once the secondary images (PET or MRI) are acquired, they are imported into our department software for image registration and fusion. Image registration is the process of aligning the parts of one image to corresponding parts in another, while image fusion relates to the combined display of imaging data.
For example, in the image above, you can find a diagnostic CT on the left, an associated PET scan in the middle, and the fused PET/CT on the right. The fused image allows your physician to view the same scan’s anatomical and functional imaging results on the CT plan for your radiation treatment.
In the example of a PET/CT, the diagnostic CT is mapped to your treatment planning CT. The PET information can be used to identify areas of concern, and your doctor can contour, or outline, these areas on the treatment planning CT during the treatment planning process.
These same tools can be used if you require future radiation treatments, either at a new site or at sites that have been treated in the past. Your old and new treatment planning CTs can be registered and fused to transfer information about your previous treatment onto the new CT scan. The transferred radiation dose information can be added to the new planned radiation dose, and your doctor will review this information to ensure that your treatment is safe and the normal tissues are spared excess radiation as much as possible.
Secondary imaging can be extremely useful for your doctor in planning your radiation therapy, though it is not required for every case. Your doctor will discuss your specific treatment plan requirements in detail.