Sci‐PM Sat ‐ 09: Respiratory gating in cancer applications, including 4‐D CT based treatment planning

Respiratory gating is one way to compensate for organs and/or tumours that are affected by respiratory motion. One approach to respiratory gating is to use the Real‐Time Position Management (RPM) Respiratory Gating System, by Varian Medical Systems, which records the cyclic motion of the chest or abdomen using fiducial skin markers while patients breathe freely. There is conflicting evidence, however, regarding the prediction of internal organ motion with external motion. Moreover, treatment planning is typically based on helical CT scans that are also acquired while patients breathe freely. Consequently, the relative position of the exterior body, the target, and the critical organs at any one phase of a breathing cycle cannot be determined accurately, and the choice of the “beam on” time could lead to error. We have developed a method in acquiring a 4‐D CT data set without the aid of external markers. Instead, the method registers images based on internal correlation at tissue interfaces between two successive respiratory phases. The 3‐D motion of the exterior body and internal organs/tumours can be derived and correlated with the RPM signal if acquired simultaneously. If strong correlation exists, then the set of images can be used for detailed treatment planning to determine the optimal treatment phase of the breathing cycle. Eighteen patients have been imaged with 4‐D CT, and one has been treated with 4‐D CT based respiratory gating. We present our results in the context of the first lungcancer patient in our clinic that was treated with this method.