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Evaluation of Long-Term Stability of KV Image Panel Position for Proton Gantry-Based Treatment Systems

M Zhu

M Zhu*, T Botticello, B Winey, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA

SU-E-T-27 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To quantify the long-term digital image panel position variation and gantry angle dependence for gantry-based proton treatment machines.

Methods: In proton gantry machines, fixed mechanical crosshairs are currently used to define beam isocenter in kV X-ray images. We developed an algorithm to automatically detect the crosshair center positions in the dual orthogonal kV images and analyzed 2651 images acquired with four image panels in the two treatment rooms within 12 months. The long-term stability of the crosshair positions in the images at four gantry angles (0, 90, 180, and 270) and the positional dependence on gantry angle were investigated. Crosshair isocentricity is analyzed bi-weekly during routine mechanical QA.

Results: The standard deviations of the crosshair positions were 0.28 mm in X-Z plane and 0.38 mm in Y axis. The 95 percentiles of the crosshair positions were -0.54 mm to 0.61 mm in X-Z plane and -0.95 mm to 0.59 mm in Y axis. The relative large variation in Y axis is caused by a systematic crosshair projection position shifts after X-ray tube and panel replacements. The crosshair position variation along gantry angles was within ±4mms, and different for each panel. The long-term crosshair position is stable and its variation is consistent to the beam isocenter QA tests, and the crosshair position variation with gantry angle is in the same pattern as measured in routine QA procedures.

Conclusion: This work revealed that the kV image panel positions in our proton treatment system is stable at the cardinal angles over 12 months and the panel positions can be correlated to gantry angles. This result indicates that the image panel position is most likely predictable and, with adequate QA flexmap procedures, the crosshair may not be necessary for all kV images.

Funding Support, Disclosures, and Conflict of Interest: The project was supported by the Federal Share of program income earned by Massachusetts General Hospital on C06 CA059267, Proton Therapy Research and Treatment Center.

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