Room: AAPM ePoster Library
Purpose: measure the influence of interfractional motion on geometric delivery accuracy in liver Stereotactic Body Radiation Therapy (SBRT) using advanced MR Imaging technique.
Methods: visualization of radiation dose deposition after liver SBRT is expected in the form of hypo-intense regions on Primovist???- MRI. We developed an algorithm to process these MRI scans, and quantified the geometric centre of mass shift between auto-segmented hypo-intensity and original target as defined on pre-treatment planning CT. The application of this approach was performed on 15 liver SBRT patients. The interfractional motion was evaluated for the same patient cohort using Cone Beam CT (CBCT) obtained at each fraction. Each CBCT was registered to the pre-treatment simulation CT to calculate the voxel-based shifts of the liver structure, which were averaged over all of the simulation CT-CBCT pairs. The interfractional motion (averaged CT-CBCT pairs) was compared against the directly evaluated delivery accuracy (CT-MRI pair) in three directions: left-right, anteroposterior and inferior-superior. The correlation coefficient (CC) was calculated to assess the impact of interfractional motion on delivery accuracy.
Results: mean (SD) interfractional motion of the liver organ for 15 patients was 2.2 (1.4), 2.4 (0.9), and 3.3 (2) mm in left-right, anteroposterior, and superior-inferior directions respectively; the overall 3D motion was 5.6 (2.8) mm. Using post-SBRT Primovist???- MRI framework, the mean (SD) delivery accuracy was quantified to be 1.5 (1.3), 2.6 (1.7), and 2 (2.7) mm in the same respective directions, and 4.4 (2.3) mm for the 3D motion. There was a weak correlation between interfractional motion and final delivery accuracy where |CC| was less than 0.25, and as low as 0.03 for the anteroposterior direction.
Conclusion: hypo-intensity on Primovist???-MRI can be used to show that the interfractional motion of the liver organ is weakly correlated to the resulting delivery accuracy of liver SBRT.