Preclinical Research Platform for Ocular FLASH Irradiation

Abstract

<h3>Purpose/Objective(s)</h3> Normal tissue sparing effects from FLASH radiotherapy (RT) are potentially transformative for the treatment of ocular cancers. Reducing damage to the abundant at-risk structures within the eye can prospectively increase visual retention rates, relax treatment margins, and eliminate the need for motion tracking by surgically implanted fiducials. To date, no prior work has explored FLASH-RT effects in the context of the eye. In this work, we have conducted a pilot study of x-ray FLASH effects for ocular irradiation in a preclinical mouse model by measuring the functional and pathological changes in healthy eyes following FLASH or conventional (CONV) dose rate irradiation. <h3>Materials/Methods</h3> An immobilization device was designed to enable focal irradiation to a single mouse eye using a FLASH-capable rotating anode x-ray tube. A 5 mm diameter lead collimator was manufactured to encompass the 3 mm target of the entire eye. Dose and dose rate measurements were performed with calibrated radiographic EBT3 films. Cone-beam CTs were acquired of four C57BL6J mice in immobilization to confirm setup reproducibility, quantified by the mean Hausdorff distance between bone segmentations. Healthy 8-week-old C57BL6J mice were irradiated with 150 kVp x-rays to doses of 21 Gy or 34 Gy at FLASH (right eye) and CONV (left eye) dose rates, respectively. Both eyes were irradiated to limit the influence of varying baseline vision between animals in our analysis. Visual acuity was assessed in 3 mice per dose level using scotopic electroretinography (ERG) up to 2 months post irradiation. Histopathological changes were assessed through H&E staining of harvested eyes. <h3>Results</h3> Mouse setup in our immobilization device was highly reproducible, with a mean Hausdorff distance of 0.34 ± 0.10 mm. Measured dose rates within the field were 67.0 ± 1.9 Gy/s and 1.2 ± 0.1 Gy/s at FLASH and CONV settings, respectively. ERGs revealed that FLASH-irradiated eyes at 21 Gy retained visual function up to 2 months post irradiation, while 21 Gy CONV induced blindness at all sampled time points with more severe surrounding skin effects. At 34 Gy, all eyes were blinded within 1 week, with comparable skin toxicities in the irradiated areas regardless of dose rate. Pathological assessment showed a loss of the photoreceptor layer of the retina from CONV irradiation, which remained intact in FLASH-treated mice. <h3>Conclusion</h3> We have developed a novel platform to study x-ray FLASH effects from ocular irradiation in mice. Functional ERG assay revealed a preservation of visual function from 21 Gy at FLASH dose rates that was not present from 21 Gy CONV. Differential damages between FLASH and CONV irradiations were confirmed through histopathology. This first demonstration of FLASH normal tissue sparing effects in a mouse eye model presents a unique and promising translation opportunity for clinical FLASH treatment. Further studies are ongoing to explore the long-term effects of FLASH radiation on vision and its efficacy on intraocular tumors.