Comparison of rigid and deformable image registration accuracy of the liver during long-term transition after proton beam therapy

Objective: To identify the previously irradiated region is important in re-irradiation. We investigated the long-term transition of geometrical accuracy of a commercially available rigid and deformable image registration (RIR, DIR) software product, MIM Maestro, in livers of patients who underwent proton beam therapy (PBT). Methods: Image registration of pre-treatment and post-treatment CT was performed in 15 liver tumor patients who received PBT and follow up post-treatment CT examination was performed over time (2-7 times each, a total of 54 times). We performed RIR and DIR of pre-treatment CT and compared the post-treatment CT by calculating the similarity of whole liver (Dice similarity coefficient: DSC) and dislocation of metallic markers implanted close to the tumor (fiducial registration error). Results: In the plain CT, the DSC was bigger with RIR in 1 case and DIR in 53 cases (0.83 ± 0.07 vs. 0.92 ± 0.04), then consistently bigger with DIR in 14 patients. The fiducial registration error was smaller with RIR in 10 cases, DIR in 43 cases, and equal error in 1 case (11.8 ± 6.9 vs. 7.6 ± 9.7 mm), then consistently smaller with DIR in 10 patients. In the contrast-enhanced CT, the DSC was consistently bigger with DIR in all 15 patients, 54 cases in all (0.84 ± 0.06 vs. 0.92 ± 0.03). The fiducial registration error was smaller with RIR in 7 cases and DIR in 47 cases (11.0 ± 6.3 vs. 6.3 ± 7.3 mm), then consistently smaller with DIR in 12 patients. Conclusion: The DIR performance of MIM is therefore superior to that of RIR and its advantage is independent the term after PBT.