Radiation oncologists have a number of methods to visualize a tumor prior to treatment, which show some kinds of information while hiding other types. Four-Dimensional Computed Tomography (4DCT) represents a major advance over traditional still imaging, in that it captures the movement of the organs as well as shows the position and shape changes of the tumor over time. However, it cannot capture the tumor metabolism information that is recorded by a positron emission tomography (PET) scan. These two technologies have been combined in Four-Dimensional PET/CT (4D PET/CT).
Four-Dimensional PET/CT (4D PET/CT) represents the latest step forward in imaging tumors in the chest, abdomen and pelvis. 4D PET/CT takes advantage of the new faster, more accurate PET and CT technology to capture the internal movement of the organs and the tumor over time, while also capturing the metabolism of the tumor. This combination gives the most complete and accurate imaging information on tumor and critical structures. Not only does this allow to get a picture of the tumor in a depth way beyond what would be possible with the eye or with CT alone - it makes it possible to see the makeup of the tumor. By seeing this information, an oncologist can see how the tumor moves with breathing and other normal body motions, and also see what parts of the tumor require more or less radiation. The result is complete, accurate treatment with the least side effects possible.
Images generated separately from 4D PET and 4DCT - click to enlarge
4D PET/CT images, showing a tumor before and after treatment
Taking a 4D PET/CT image is similar to traditional PET/CT scanning. In the procedure, the patient lies on his or her back. 4D PET and 4D CT images are then acquired one after the other while on the same table. The complete sequential acquisition of 4D PET and 4DCT images takes about 15-25 minutes. After the images are acquired, they are fused together. These images show how the tumor moves or is affected by breathing and moving organs nearby (as illustrated by the pictures above).
The information gathered from 4D PET/CT scan allows for radiation to be delivered to the tumor within a certain interval in the breathing cycle. This is done using gated IMRT, a form of IMRT where the radiation is stopped when the tumor is not in the correct position. This allows the use of higher, more effective levels of radiation while minimizing the impact on healthy tissue, making new levels of treatment possible.
