 Hello everyone. Here is a shared study with title comparison of dosimetric impact of intrafractional setup discrepancy between multiple and single isocenter approaches in leaner-based stereotactic radiotherapy of multiple brain metastases. Let's have a brief introduction first. Brain metastases are the most common presentation of intracranial tumors. Approximately two-thirds of patients were presented with multiple lesions. In leaner-based SRT, previous studies have demonstrated comparable dosimetric outcomes but improved efficiency by a single isocenter approach, instead of typically used multiple isocenter approach. However, the difference in the extent of impact brought by intrafractional shifts to both approaches has not been fully evaluated. Therefore, the purposes of this study were to evaluate the dosimetric results of SI and MI approaches. And to compare the effects of intrafractional setup discrepancies on the robustness of the respective approach. 22 patients with 2-3 brain metastases were retrospectively recruited. A total of 46 lesions were identified. MI and SI plans were generated using the Eclipse system, with non-coplanar static IMRT beams and micro-MLCs. The acceptance criteria for plans with at least 98% of PTV was covered by the prescribed isodose line with maximized paddock CI achieved and acceptable OARs doses. Here is an illustration of isocenter location and dose distribution in plan using the two approaches respectively. To simulate the intrafractional errors, 16 types of isocenter displacements, including AP, LR, SI translations, and roll, yaw and pitch rotations, were hypothetically applied to each original plan of both approaches. Translational shifts within 2 mm with every 0.5 mm increment, and rotational shifts within 2 degree with every 0.5 degree increment were tested. The dose coverage of PTV was evaluated by paddock CI and volume of regret. The following are the results for Part 1 of the study. In terms of target conformity, mean CI of SI plans, 0.83, was comparable to that of the MI plans, 0.84. For the OARs doses, majority of them showed higher mean D max in the SI plans except for the brainstem. But the overall dosimetric deviations between the two approaches were relatively subtle. The next part is the results regarding the impact of isocenter shifts. With regard to the PTV, change of CI for all magnitudes of translational shifts were similar. Instead, for the rotational shifts, the SI plans showed significantly greater change of CI after all extents of shifts, with the differences being exaggerated particularly after 1.5 degree and 2 degree shifts. The figure shows that deviations between the prescribed blue isodose line and the red PTV contour lines were further magnified along increased magnitude of shifts in SI plans, whereas that of MI plans could be kept more stable. The graphs here show generally a pattern of decreasing robustness of SI plans as the PTV isocenter distance increased. Their regression lines indicated a recommended threshold distance of 3.6 to 3.7 centimeters for 2 degree shifts, assuming a maximum allowable change of CI is 0.2. Using the SI approach, a small angular error could result in considerable dosimetric degradation particularly for small targets situated further away from the treatment isocenter. For the OARs, the impact of all shifts was relatively mild with change of D max substantially less than 1 gray in both MI and SI plans. There is no definite pattern that any of the treatment approaches would be favored. All in all, to conclude, both MI and SI approaches could produce clinically acceptable plans. But there were dosimetric impacts of isocenter shifts, with SI plans being more vulnerable in larger magnitude of rotational shifts. It is crucial to minimize the intrafractional setup discrepancies for the potential clinical benefit gaining from the SI approach. Further research for investigating the respective clinical consequences is suggested. Thank you.