Abstract A003: Influence of beam pauses, dose, and dose rate on the FLASH effect in two mouse models
Authors: Johnson Erickson Danielle P. , Cui Sunan , Cao Ning , Ruth Matt , Morimoto Alec , Shaver Benjamin A , Seitz Zacharias , Grassberger Clemens , Saini Jatinder , Wong Tony , Ford Eric , Rengan Ramesh , Zeng Jing , Schwarz Marco 
Published: January 2025

Abstract
Limited understanding of the FLASH effect poses a challenge in gauging the feasibility of translation to clinical practice with the available radiotherapy equipment. We aimed to examine the effect of pausing during treatment, whether due to multi-field treatments or technical difficulties, and if there is a minimum dose or dose rate threshold for FLASH. Experiments were conducted on two distinct beamlines with associated beam properties: (i)a preclinical beamline at University of Washington Medical Center (UWMC) delivering a 50 MeV proton beam in scattering mode, and (ii)a clinical beamline at Fred Hutchinson Proton Therapy Center (PTC), delivering 229 MeV protons with pencil beam scanning (PBS). After dose finding experiments, we delivered a total 14-16 Gy to the pelvis of BALB/c mice or 6-8 Gy to the pelvis of SCID mice in transmission mode (∼45-120 Gy/s FLASH, 0.5-1 Gy/s CONV), using survival as endpoint. Utilizing both beamlines, we demonstrated a FLASH effect for delivery without pauses at or above 45 Gy/s, improving survival from 45% to 83%. The addition of a 30 second- or 2-minute pause in delivery, resulting in 2 doses of 7-8Gy delivered at 45Gy/sec or higher, resulted in a reduction of the FLASH effect, with an overall survival of 67% with either pause length. This led to question if the reduced effectiveness was due to the pause in treatment or reaching a minimum dose threshold. We then chose to examine FLASH with single 6-8Gy exposures in SCID mice as they have a much higher radiosensitivity and enable us to study the same endpoint at lower dose. Interestingly, in SCID mice we did not see a FLASH effect at all, with no significantly different survival either at 8Gy (78-80% survival) or 6Gy (12-15% survival) between FLASH and CONV arms. This loss of the FLASH effect could be explained by several factors; first, we may have hit the minimum dose threshold, second, SCID mice have impaired DNA damage repair (DDR), and third, SCID mice lack a fully functional immune system. To tease this apart we are currently performing experiments to deplete T-cell populations in BALB/c mice with anti-CD4/CD8 antibodies prior to irradiation to reduce immune system functionality but maintain DDR. Results are forthcoming. We observed the FLASH effect for both scattering and scanning proton beams when the dose was delivered in a single irradiation at ≥45 Gy/s. Including a pause of 30 seconds or 2 minutes seems to attenuate the FLASH effect but does not fully negate it. This attenuation may be due to the pause itself or the existence of a minimum dose threshold. Additionally, we have observed a loss of the FLASH effect in SCID mice, but the cause of this remains unknown and under investigation. At present we are working to decouple the potential effects of DDR and immune response in the FLASH effect in SCID mice.
Citation Format: Danielle P. Johnson Erickson, Sunan Cui, Ning Cao, Matt Ruth, Alec Morimoto, Benjamin A Shaver, Zacharias Seitz, Clemens Grassberger, Jatinder Saini, Tony Wong, Eric Ford, Ramesh Rengan, Jing Zeng, Marco Schwarz. Influence of beam pauses, dose, and dose rate on the FLASH effect in two mouse models. [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Translating Targeted Therapies in Combination with Radiotherapy; 2025 Jan 26-29; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(2_Suppl):Abstract nr A003.