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Semi-Automated IGRT QA Using a Cone-Shaped Scintillator Screen Detector for Proton Pencil Beam Scanning Treatments

W Cai

W Cai1,2*, H Oesten2 , B Clasie2 , B Winey2 , K Jee2 , (1) Brigham and Women's Hospital and Harvard Medical School, Boston, MA, (2) Massachusetts General Hospital and Harvard Medical School, Boston, MA


MO-L-GePD-JT-1 (Monday, July 31, 2017) 1:15 PM - 1:45 PM Room: Joint Imaging-Therapy ePoster Theater

Purpose: In order to promote accurate image-guided radiotherapy (IGRT) for a pencil beam scanning (PBS) proton system, a new QA procedure employing a cone-shaped scintillator screen detector has been developed for multiple QA tasks in a semi-automatic manner.

Methods: The cone-shaped scintillator detector (XRV-124, Logos Systems, CA) is sensitive to both x-ray and proton beams, and records its scintillations on the cone surface as a 2D image, from which the geometrical information of the radiation field that enters and exits the cone surface can be potentially extracted. Utilizing this feature, QA parameters that are essential to PBS IGRT treatment were measured and analyzed. The first application was to characterize the cross hair accuracy of onboard x-ray imaging systems, where the gantry angle was calculated based on the x-cross hair, the position of the y-cross hair was obtained from the radius of its circular shadow, and the offset of the cross hair center was calculated from the calibrated geometric center. The second application was to measure the position and size of pencil beam spots using an algorithm that recovers the actual pencil beam profiles from the raw detector images. These applications allowed the checks of the coincidence of laser, imaging and radiation isocenters, and the dependency of gantry-angle and beam energy for pencil beam positions and profiles.

Results: The results of gantry angle were validated against the mechanical readings and an agreement of 0.3 degree was achieved. The cross-hair offsets were validated against the laser isocenter, and an agreement of 0.3mm was achieved. The sigma values of pencil beam were validated against MatriXX measurements and an agreement of 0.5mm was achieved.

Conclusion: The cone-shaped scintillator system showed advantages in making multi-purposed measurements with a single setup. The in-house algorithms were successfully implemented to analyze key QA parameters for a PBS system.

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