Software-controlled, highly automated intrafraction prostate motion correction with intrafraction stereographic targeting: System description and clinical results

Purpose: A new system for software-controlled, highly automated correction of intrafraction prostate motion, "intrafraction stereographic targeting" (iSGT), is described and evaluated. Methods: At our institute, daily prostate positioning is routinely performed at the start of treatment beam using stereographic targeting (SGT). iSGT was implemented by extension of the SGT software to facilitate fast and accurate intrafraction motion corrections with minimal user interaction. iSGT entails megavoltage (MV) image acquisitions with the first segment of selected IMRT beams, automatic registration of implanted markers, followed by remote couch repositioning to correct for intrafraction motion above a predefined threshold, prior to delivery of the remaining segments. For a group of 120 patients, iSGT with corrections for two nearly lateral beams was evaluated in terms of workload and impact on effective intrafraction displacements in the sagittal plane. Results: SDs of systematic (∑) and random (σ) displacements relative to the planning CT measured directly after initial SGT setup correction were <0.5 and <0.8 mm, respectively. Without iSGT corrections, effective ∑ and σ for the 11-min treatments would increase to ∑eff <1.1 mm and σeff <1.2 mm. With the iSGT procedure with an action level of 4 mm, effective positioning errors were reduced to ∑eff <0.8 mm and σeff <1.0 mm, with 23.1% of all fractions requiring a correction. Computer simulations demonstrated that with an action level of 2 mm, the errors would have been reduced to ∑eff <0.6 mm and σeff <0.7 mm, requiring corrections in 82.4% of the fractions. Because iSGT is highly automated, the extra time added by iSGT is <30 s if a correction is required. Conclusions: Without increasing imaging dose, iSGT successfully reduces intrafraction prostate motion with minimal workload and increase in fraction time. An action level of 2 mm is recommended.

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