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Session: Proton Therapy I [Return to Session]

Characterization of An Isocenter Alignment Device for the Proton PBS Dynamic Collimation System

T Geoghegan1*, K Patwardhan1, N Nelson3, L Bennett 1, J Yu2, A Gutierrez2, P Hill3, R Flynn1, D Hyer1, (1) University of Iowa, Iowa City, IA, (2) Miami Cancer Institute, Miami, FL, (3) University of Wisconsin, Madison, WI


SU-H330-IePD-F4-3 (Sunday, 7/10/2022) 3:30 PM - 4:00 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 4

Purpose: To test and characterize a camera and scintillator based isocenter alignment device (IAD) for the proton Dynamic Collimation System (DCS). The DCS is a tray mounted accessory that can be quickly removed and installed from the proton nozzle and provides sharpening of the lateral penumbra. While ease of installation is paramount in its clinical utility, accurate alignment between the DCS and the radiation isocenter must be ensured prior to use.

Methods: The IAD attaches to the base of the DCS. Within the light-tight, aluminum box that defines the exterior of the IAD, a 12.3-megapixel Sony IMX477 CMOS sensor with a 6 mm lens monitors a Gadox (Gd2O2S:Tb, P43) scintillating screen via a mirror. Radiation isocenter alignment begins by individually driving each axis to the mechanical isocenter of the DCS and delivering a uniform proton field (7x7 cm2) while simultaneously acquiring a long exposure image of the Gadox screen. In the resulting image, the shadow of the trimmer can be clearly seen. The center of the trimmer is compared to the center of the radiation field to determine the offset between the radiation and mechanical isocenter of the DCS. This offset is then passed to the DCS control system and the mechanical DCS coordinate system is re-calibrated to match the radiation isocenter.

Results: The absolute accuracy of the IAD was measured to be on average 77 µm with a standard deviation of 53 µm, when compared to centering calibration via the inline x-ray imager. The reproducibility, taking multiple exposures and of the same trimmer position and recalculating the offset, was measured to be within a range of 150 µm with a standard deviation of 73 µm.

Conclusion: The IAD allowed alignment of the DCS to be completed quickly and accurately with a single beam delivery for each axis.

Funding Support, Disclosures, and Conflict of Interest: Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R37CA226518. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.


Protons, Collimation, Quality Assurance


TH- External Beam- Particle/high LET therapy: Proton therapy – quality assurance

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