Purpose: To compare the image guided registration errors (3D vs pseudo 6D) between the planning CT and the cone beam CT (CBCT) for intensity-modulated radiotherapy (IMRT) in nasopharyngeal carcinoma (NPC); and investigate the dosimetry differences between the two registration methods.
Methods: A pseudo 6D registration could be performed without an 6D couch when only the translational shifts were applied. Firstly, a total of 40 NPC patients were collected with the same immobilization device to reduce the localization error in the radiotherapy. Secondly, Both planning CT and CBCT were registered online on the Linac console via 3D and 6D registration, respectively. Thirdly, two sets of positioning errors/shifts were obtained to explore the differences. Finally, the 3D error and pseudo-6D error plans were simulated and re-calculated via the Eclipse planning system to investigate the dosimetry differences, including both Target and OARs dosimetric values.
Results: There was no statistically significant difference between 3D error and pseudo 6D set-up error, (P>0.05). In translational shifts, 3D set-up error （x±s）were ₋₋₋ X:0.28±1.12 mm, Y:0.21±0.85 mm, Z:0.25±1.03 mm; and pseudo 6D set-up error （x±s）were ₋₋₋ X:0.22±0.93 mm, Y:0.15±0.62 mm, Z:0.31±1.27 mm, respectively. There was also no significant difference between Conformity Index (CI) and Homogeneity Index (HI), respectively. Both Target and OAR were compiled the clinical requirements.
Conclusion: In NPC image-guided intensity-modulated radiotherapy, the 6D rotation error had little effect on the change of 3D translation error, and there was no difference between the 3D and pseudo 6D set-up error plans in the target conformity index and uniformity index.