Purpose: The CyberKnife Synchrony™ respiratory tracking system uses external markers in conjunction with a pair of planar x-ray images to track tumor motion in real time. However, x-ray imaging requires either implanted fiducials or an unobstructed view of a high-contrast tumor. If the patient cannot tolerate a fiducial implant and the tumor cannot be visualized with planar images, our current departmental procedure is to treat the patient on a Varian Truebeam™ directly aligning the tumor on the CBCT and limiting tumor motion with a compression belt. The purpose of this project is to evaluate whether the belt reproducibly reduces breathing motion and fixes ITV-position for treatment on the CyberKnife with limited PTV margins and spine-tracking.
Methods: CBCT images for each of the 5 or 10 lung SBRT fractions treated on the Truebeam were exported from the MOSAIQ™ radiation oncology information system to the MIM Maestro™ software. The RT-structure sets defined in the Pinnacle™ TPS on the planning CT were copied and adjusted to each CBCT using the MIM Adaptive Re-Contouring tool. The CBCT was then rigidly aligned on the planning CT with an ROI box on the spine to mimic the CyberKnife spine-alignment. Finally, four metrics provided in the MIM deformable image registration tool were utilized in the evaluation of CBCT-to-CT contour similarity, the Hausdorff Distance (HD), Mean Distance to Agreement (MDA), Dice Similarity Coefficient (DSC), and Jaccard indices.
Results: The metrics analyzed for the ITV and averaged over all fractions per course and an initial dataset of five patients was HD 5.4±1.1 mm, MDA 1.6±0.5 mm, DSC 0.77±0.08, and Jaccard 0.64±0.1. The DSC was higher for ITVs in the lower lope than in the upper lope.
Conclusion: The preliminary results suggest that belt-compression in conjunction with CK-spine-tracking is a viable option using the common 3-5mm ITV to PTV expansion.
TH- RT Interfraction Motion Management: setup errors, immobilization, localization