Room: ePoster Forums
Purpose: To evaluate the effectiveness of 1-level and 2-level plate calibration, and megavoltage (MV) cube calibration for optical surface imaging (OSI) systems to reduce the couch-angle dependency (CAD) error for surface-guided intracranial stereotactic radiosurgery (SRS) or radiotherapy (SRT), which usually involves couch rotations with tolerance of 1mm.
Methods: Three calibration procedures were evaluated, including the 1-level (2D, at isocenter level) and 2-level (3D, at isocenter level and 7.5-10cm above) plate calibration, and MV-OSI isocenter concordance calibration. The effectiveness of these calibrations to reduce CAD error was evaluated for intracranial frameless SRS/SRT treatments by performing a CAD test after each calibration. In the CAD test, an anthropomorphic head phantom was immobilized on the treatment couch with the middle of the brain aligned with the radiation isocenter, indicated by the room lasers. An on-site reference surface image was captured at couch zero and a region of interest was drawn similar to the uncovered area in an open-face mask. The couch was rotated with 10Â°ï‚° interval up to Â±90Â°, and the residual registration error was recorded as the CAD error. OSI baseline drift (OBD) over 20 min was evaluated under the same condition at couch zero. Ten OSI systems (5 are capable of MV cube calibration) in five clinical centers were evaluated by seven trained medical physicists.
Results: The average maximum CAD error of the 10 systems was reduced from 1.1Â±0.5mm to 0.7Â±0.2mm from 2D to 3D plate calibration. The MV cube calibration after 2-level did not further reduce the CAD error (0.9Â±0.4mm). The OBD error plateaued at 0.3mm after 10min.
Conclusion: The 2-level plate 3D calibration produces smallest CAD error. The isocenter fine-tuning using the MV cube provides verification, but may not further reduce the CAD error. The CAD and OBD tests are recommended for OSI system commissioning in SRS/SRT treatment.