2424 - Preclinical FLASH-SARRP System: Initial Experience on Small Animal Setup and Workflow

D. Sforza¹, M. Tajik-Mansoury¹, J. W. Wong¹, I. Iordachita², and M. Rezaee¹; ¹Department of Radiation Oncology and Molecular Radiation Sciences, School of Medicine, Johns Hopkins University, Baltimore, MD, ²Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD

Purpose/Objective(s): A novel self-shielded preclinical FLASH x-ray cabinet, FLASH-SARRP, has recently developed at Johns Hopkins University for in vivo and in vitro study of FLASH effects. Here, we characterize dosimetric commissioning of the system and develop image-guided animal setup for head and neck, thorax, and abdomen irradiation of a mouse subject. Materials/

Methods: FLASH-SARRP utilizes two high-power rotating anode x-ray sources with 150 kVp potential installed in parallel-opposed geometry. To achieve ultrahigh dose-rates (> 40 Gy/s), 30 mm separation distance was set between x-ray tube housings. This distance is sufficient to accommodate a 20-mm thick phantom, representing a mouse thickness, and 5-mm working space between the phantom surfaces and tube housings. Dosimetric performance was evaluated using in-air and in-phantom measurement by calibrated ion chambers, TLD-100 dosimeter and EBT3/XD films. Animal setup was implemented utilizing custom-designed (in-house) micro-CT as offline 3D imaging system for FLASH-SARRP. Docking system with different beds was designed for mice imaging and irradiation based on required field dimension and irradiation sites including eye, brain, tongue, skin, lung, and pancreas. The docking system could accommodate mice with reproducible immobilization using robber bands for extremities and tail with an adjustable bite-bar and nose cone including a thumbscrew locking mechanism. The bed also included multiple holes for fiducial insertion. The design of docking system supported integration of the bed with immobilized mouse into both the micro-CT and FLASH-SARRP systems. A single x-ray source was utilized to image the mouse on radiographic films for comparison with digitally reconstructed radiographs (DRRs) from micro-CT. Double exposure 2D imaging technique was utilized for the verification of radiation field placement.
Results: FLASH and conventional dose rates at the standard animal setup was in ranges of 40–110 Gy/s and 0.5–2.8 Gy/s respectively. Beam output was highly linear (r²>0.99) with exposure time and current, allowing for precise adjustment of dose and dose-rate. Negligible discrepancy (< 1 mm) in the distance between bony landmarks and fiducials was found in the transfer of docking system with immobilized mouse from micro-CT to FLASH-SARRP. These results suggest robustness of the docking system for mouse positioning and immobilization across different irradiation sites. Total thickness of immobilized mouse with bed and docking system was in the range of 15 – 20 mm, depending on the age of mouse, which supported animal setup within 30 mm separation distance between two x-ray tube housings.
Conclusion: FLASH-SARRP system have desirable dosimetric performance for small animal irradiation at both FLASH and conventional dose rate irradiation. Inter-comparison of DRR from micro-CT and radiographic films from FLASH-SARRP verified robustness of the docking system for accurate and reproducible positioning of the mouse in the system.