K Patwardhan*, T Geoghegan, R Flynn, D Hyer, University of Iowa Hospitals and Clinics, Iowa City, IA

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  K Patwardhan

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PO-GePV-T-184 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To develop an automated mechanical calibration protocol for the linear positioners used in the Dynamic Collimation System (DCS), an aftermarket collimator for Pencil Beam Scanning (PBS) proton therapy. The linear positioners are used to move the four trimmer blades of the DCS accurately and precisely before each spot delivery to maximize the improvement in lateral dose falloff.

Methods: A DCS prototype was built consisting of two pairs of orthogonal nickel trimmers independently translated by Parker T1S linear positioners. The linear positioners are equipped with encoders that provide spatial positioning feedback. The DCS was designed with calibration blocks at known positions to calibrate the encoders each time the system is powered on. To accomplish this calibration, each trimmer is slowly translated across the field-of-view until the trimmer makes physical contact with the calibration block. The electrical current provided to each linear positioner is monitored using the control system and contact with the calibration block is detected via a spike in the current demand. Because the position of the calibration block and the mechanical origin of the DCS is well known, this technique can be used to define the mechanical coordinate system for the device. A 1x1 cm² field was created using the trimmers and the distance between the trimmer faces was measured before and after calibration.

Results: Prior to calibration, a maximum discrepancy of 4 mm was observed in the mechanical positioning of trimmers as compared to 0.02 mm after calibration. The calibration routine ensures a repeatable and robust definition of the mechanical coordinate system.

Conclusion: The DCS linear positioners can be automatically calibrated to a mechanical coordinate system. Ultimately, this calibration can be translated to the radiation isocenter coordinate system via known offsets, allowing the DCS to be efficiently used for collimated PBS treatments.

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.

Keywords

Protons, Beam Shaping, Quality Assurance

Taxonomy

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

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