Geometric Rectification of 6DoF Cbcts: Streamlining Dose Calculation In Commercial TPS
Abstract
Purpose
Commercial treatment planning systems (TPS), such as Varian Eclipse, cannot perform dose calculations on CBCTs acquired with 6DoF couch corrections. This limitation prevents direct dosimetric re-evaluation of verification scans. This study presents an offline workflow to rectify tilted geometries and generate "pseudo-matched" CBCTs to enable TPS-integrated dose calculation for offline adaptive planning.
Methods
The workflow was developed using Python and SimpleITK. Tilted CBCT volumes are rectified using inverse transformation matrices derived from DICOM headers, aligning them with the standard axial coordinate system. To enhance clinical efficiency, a web-based UI with ODBC connections to the MOSAIQ Record and Verify system was implemented. This allows users to select CBCTs and execute the pipeline without manual file exports. The tool also supports generating synthetic images by applying planned couch shifts based on the Spatial Registration Object (SRO) to the CBCT for "what-if" scenarios. Validation included phantom cases with known 6DoF rotations and clinical datasets.
Results
Rectified and pseudo-matched images were verified for geometric accuracy in MOSAIQ and Eclipse, confirming precise alignment with intended shifts and the planning coordinate space. Spatial uncertainty was sub-voxel, with mean 3D deviations < 0.5 mm compared to phantom ground truth. Total processing time, including MOSAIQ data retrieval, is under 30 seconds per dataset. The automation significantly reduced manual intervention, providing an efficient solution for preparing previously incompatible couch-rotated images for analysis.
Conclusion
This workflow overcomes TPS geometric constraints regarding couch rotations. By integrating rectification with an ODBC-linked interface, it provides a robust, accessible pathway for offline adaptive radiotherapy and dose re-evaluation.