Purpose: Synthetic CT (sCT) generated by deformable image registration (DIR) of planning CT to CBCT was proposed for the delivered dose calculation in adaptive radiation therapy (ART). We aim to evaluate the registration accuracy when the acquisition times of the two images are different (patient anatomy has changed).
Methods: A patient who had replanning CT in his abdomen due to significant weight loss during radiation treatment was chosen. Two sCT images were generated by deformable image registration (DIR) in Velocity using (1) initial planning CT (sCT1) and (2) replanning CT (sCT2), where planning CT images were deformed to CBCT that was taken on the same day of replanning CT. The DIR was performed using rigid and deformable multi pass option in the Velocity software system. After DIR, deformation vector and anatomical alignment were visually inspected. The deformed images were further analyzed for HU difference.
Results: The magnitude of the difference between the two planning CT images was not fully realized between sCT1 and sCT2 due to the registration of initial CT to CBCT of a later day. A visual evaluation was satisfying except that the two sCT images showed a slight disagreement on external body contour and a more visible disagreement in the location of air in the internal organ. The two sCTs showed acceptable agreement in HU values as shown by the standard deviation of the HU difference of 54.5 HU centered around 0, across the entire voxels of the images.
Conclusion: Using initial planning CT for sCT generation is acceptable when a patient's anatomy changes significantly in the abdomen. However, one should be aware of the possibility of losing the accuracy in the external outline and details of internal organs. Further dosimetry study is needed.