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Abstract: Purpose: Periodic motion of the target can be compensated by translational motion of the treatment beams in robotic SBRT. However, spontaneous, non-periodic displacement of the target may completely change the treatment geometry. In this case, translation is not sufficient since relative motion between the PTV and OARs can cause substantial deviations of dose in the OARs. Instead, solving a new optimization problem is required after partial dose delivery. We demonstrate this effect and propose a method for adaption by replanning which accounts for the change in the geometry. Methods:In contrast to typical adaptive strategies, our approach is based on complete and constrained replanning of the optimization problem which guarantees that no side effects such as higher doses than prescribed can occur in the treatment plan. We adapt the linear program to account for the changed treatment geometry which allows for fast reoptimization. For evaluation, we translate the target with random direction and length sampled from a truncated normal distribution with mean values from 12.5 to 30mm without overlap with OARs. We stu dy treatment plans with approximately 300 treatment beams and consider the motion to occur after 100 delivered beams. We solve in total 40,950 inverse planning problems for 45 patients. Re sults: Replanning can compensate for coverage loss and avoid constraint violation. Runtime of reoptimization is on average 14s. When not compensating for movement, coverage can decrease from 95% to 20%. While translation of the beam source can compensate for loss in coverage, dose constraints can be violated. E.g. maximum dose in the rectum is violated in 62% of treatment plans with translational compensation. Conclusion: For non-periodic target displacements, translational compensation can lead to suboptimal treatment plan delivery. Constrained replanning after partially delivery of the treatment plan can compensate for the negative impact on the delivered dose distribution