Purpose: The purpose of this work is to design and validate an isocenter alignment system for the Dynamic Collimation System (DCS), a proton collimator comprised of four nickel trimmers that move in synchrony with a scanned proton beam to sharpen the lateral penumbra. Accurate alignment of the DCS with the radiation isocenter is necessary as the lateral dose distribution is sensitive to the spatial relationship between the proton beam and the collimator.
Methods: An alignment system was designed to be affixed to the base of the DCS and consists of a chromium-doped alumina scintillating screen (Chromox), a mirror, and a Raspberry Pi camera system. The camera system, which is mounted outside of the primary beam, views the scintillator via the mirror which is tilted at 45°. Each axis is aligned to radiation isocenter by irradiating a nickel trimmer with a uniform, 150 MeV scanned proton field (7x7 cm²) while acquiring a long exposure image of the Chromox screen. From the acquired images, the center of the trimmer is compared with the center of the radiation beam to determine the offset between the radiation and mechanical isocenter of the DCS. This calculated offset is passed along to the DCS control system such that the mechanical DCS coordinate system can be re-calibrated to match the radiation isocenter. This process is then repeated for each axis.
Results: The alignment system was able to calculate the radiation isocenter offset within 1.1 mm as compared to a beams-eye-view x-ray image.
Conclusion: Initial results indicate that the camera system is capable of aligning the trimmers to the center of a scanned proton field. Automation and further refinement of this system could result in an efficient QA alignment platform for the DCS
Funding Support, Disclosures, and Conflict of Interest: Research reported in this abstract was supported by the National Cancer Institute of the National Institutes of Health under award number R37CA226518. Hyer and Flynn are co-inventors on a patent that has been licensed to IBA.