Author: Lily Jo Bertemes, Careesa Billante, Ashley Cetnar, Arnab Chakravarti, Nilendu Gupta, Sagarika Jain, Maximilian Stephen Meineke, Allison N. Palmiero, Runhe Tan 👨🔬
Affiliation: The Ohio State University - James Cancer Hospital, Department of Radiation Oncology, The Ohio State University Wexner Medical Center, Department of Radiation Oncology, Ohio State University Comprehensive Cancer Center, The Ohio State University, The James Cancer Center 🌍
Purpose: Ultra-High Dose Rate Radiation Therapy (UHDR-RT) induces normal tissue sparing while maintaining iso-effective tumor control compared to conventional methods, known as the “FLASH effect”. However, literature suggests this effect reduces with increasing field size. A potential solution is spatial fractionation via small field grid collimation.
Methods: A copper electron cutout grid collimator was fabricated for the Varian Clinac iX with Flash Research Extension (FLEX) for non-clinical use, featuring four 2.2cm diameter holes 2.6cm apart (center-to-center). A 16MeV UHDR beam was generated with 4.2µs pulse width and 180Hz pulse repetition frequency, corresponding to a dose per pulse of approximately 100cGy at Dmax. Profiles and percent depth dose (PDD) curves were obtained with Gafchromic EBT-XD film for grid and open fields at 101cm SSD. PDD films were taken in water with a film holder oriented parallel to the beam axis. Profile films were measured perpendicular to the beam axis in solid water at depths between 0.1-3cm for peak-to-valley ratio calculation. A scanning ion chamber in a water tank was used to collect PDDs and profiles at conventional dose rates.
Results: Depths of 100%, 90%, 80%, and 50% of maximum dose were 1.12cm, 2.81cm, 3.35cm, and 4.87cm with the grid collimator. R50 for the grid beam was 1.67cm less than the 6cm open field’s R50 of 5.0cm. The peak-to-valley ratio of the grid collimator was largest at Dmax, with the normalized valley dose at 19% of the peak dose. The dose under the blocked region between four holes relative to the open field was 21.8% with output factor of 0.95 of the grid holes relative to the open field.
Conclusion: Spatial fractionation for UHDR electron beams was characterized for preclinical experimentation. Grid optimization via Monte Carlo models and large animal testing is ongoing, enabling investigations of potential synergy between grid and UHDR-RT.