D Miles*, D Sforza, J Wong, M Rezaee, Johns Hopkins University, School of Medicine, Department of Radiation Oncology and Molecular Radiation Sciences, Baltimore, MD

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  D Miles

SU-E-BRC-2 (Sunday, 7/10/2022) 1:00 PM - 2:00 PM [Eastern Time (GMT-4)]

Ballroom C

Purpose: The shallow depths and small target volumes of ocular tumors provide an advantageous site to explore the translation of FLASH radiation treatment. Here we present the development of a platform to study FLASH effects from ocular irradiations in mice with orthovoltage x-rays.

Methods: A 3D printed immobilization platform was designed to reproducibly fix the head of a mouse at 46mm away from the focal spot of a FLASH-capable rotating anode x-ray tube, operating at 150 kVp with 0.025mm Cu added filtration. To assess setup reproducibility, cone-beam CTs of four healthy immobilized C57Bl6J mice were acquired. Registered images were used to calculate the mean Hausdorff distances between bone segmentations as a metric of setup uncertainty. Lead collimators with appropriate margins were then designed for each eye and docked into the immobilization platform. Output and beam profiles as a function of depth in kV solid water were measured with calibrated Gafchromic EBT3 film.

Results: The mean Hausdorff distance between bony features was 0.34±0.08mm. A 5mm field size was used to encompass the entire mouse eye (3mm) with additional 1-mm margin. The surface dose rate measurements for FLASH and conventional settings were 67.9 ± 1.9 and 1.2 ± 0.1 Gy/s, respectively. Beam flatness at the phantom surface is within 4% of unity and the 80%-20% penumbra dose widths are less than 0.4mm. The PDD falls to 68% at 5-mm depth, reducing dose rates to 45.5 and 0.8 Gy/s at FLASH and conventional settings, respectively.

Conclusion: The described platform is capable of precision setup and irradiation of small fields on mice. Dose rates remain above 40 Gy/s for the proximal 5mm in solid water, sufficient for murine ocular irradiation. Preliminary in vivo studies with this system are currently exploring vision retention following a high-dose single fraction of FLASH or CONV x-rays.

Funding Support, Disclosures, and Conflict of Interest: This work was funded by the ASTRO-AAPM Physics Resident/Post-Doctoral Fellow Seed Grant.

Keywords

Not Applicable / None Entered.

Taxonomy

TH- Small Animal RT: Development (new technology and techniques)

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