Four-dimensional computed tomography

Respiratory motion imposes a significant challenge in radiation therapy of the lung tumor. Because most of the patients are treated in free-breathing, the information of tumor motion is critical for the radiation oncologist to delineate the target tumor. Prior to the four-dimensional computed tomography (4D-CT) imaging of the lung tumor, one or more CT scans were taken to ensure sufficient sampling of the target tumor at various phases of a respiratory cycle. This may include a free-breathing CT (FB-CT) scan plus a couple of breath-hold CT (BH-CT) scans at the end-inspiration and end-expiration phases. This practice could be problematic because a FB-CT scan can distort the shape of the target tumor. An example is shown in Figure 7.1. This is a problem even with the fast gantry rotation cycle time of less than 1 s on a modern CT scanner. A study by Underberg et al. ¹ demonstrated that a single 4DCT scan could encompass a significantly larger internal target volume than 6 CT scans did in 2 of the 10 patients of stage I non-small-cell lung cancer, whose tumors exhibited the greatest mobility. Some suggested that a 174slow CT scan of 4 s per revolution can be used to assess the tumor motion. ² However, the 4-s scan can introduce some severe motion artifacts when compared with the average CT, averaged from the multiple phases of 4DCT as shown in Figure 7.2.