B. A. Insley1, D. A. Bartkoski2, P. Balter3, S. Prajapati4, R. C. Tailor5, D. A. Jaffray6, and M. R. Salehpour3; 1The University of Texas MD Anderson Cancer Center, HOUSTON, TX, 2Empyrean Medical Systems, Boca Raton, FL, United States, 3Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 4Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 5The University of Texas MD Anderson Cancer Center, Houston, TX, 6MD Anderson Cancer Center, Houston, TX
Purpose/Objective(s): Perform Monte Carlo simulations to understand the dosimetric performance of a novel converging beam small animal irradiator using theoretical compact X-ray tube previously reported in the literature by Insley et al. (Medical Physics, 2023). Materials/
Methods: The X-ray tube comprises a compact, carbon nanotube-based 200-kV tube optimized for high intensity and directionality using ultrathin conical transmission diamond-tungsten target. We have designed a multi-source, converging irradiator platform to place 52 of these sources uniformly around a spherical collimating shell to generate conformal and arbitrary dose distributions within small animals for preclinical research. A simulation toolkit TOPAS Monte Carlo codes were employed to simulate the dosimetric performance of the platform for various collimating configurations and source settings (including focal spot size setting and source to axis distance). Dose rate distributions within a 4-cm diameter sphere were calculated for 110 different combinations of X-ray tube focal spot size, source to axis distance, collimator geometry, and field size. Results: Spherical dose spots were calculated with a large range of dose rates, field sizes, and penumbrae. Maximum dose rates >80 Gy/s (pulse averaged) were calculated for the largest focal spot size at field sizes greater than 4 cm. FLASH dose rates (>40 Gy/s) were calculated for field sizes down to 2.5 cm FWHM. Conventional dose rates (600 cGy/min minimum) were calculated for 1-mm field sizes, and penumbrae as low as 18% of the FWHM were demonstrated. Additionally, preliminary KERMA simulations demonstrate the feasibility of a 0.1-mm field size. Leakage and scatter simulations report a minimum shielding thickness of 8 mm of lead in an 80-cm cube based on workload estimates from SARRP. This platform design features a large parameter space, offering 52 individual X-ray sources with variable focal spot size (9 different choices), tube current (up to a maximum of 100 mA for the largest focal spot size), source to axis distance (between 7 and 20 cm), field size (10, 1, or 0.1 mm), and collimation geometry (3 choices of collimators for each focal spot size and field size, with selections weighted towards either high intensity, high precision, or high field flatness). The theoretical platform requires a peak power of 520 kW, which is between the average power of a CT and a low-power LINAC. Conclusion: Monte Carlo simulations of this multi-source, converging beam small animal irradiator demonstrate the potential to greatly expand the reach of preclinical research into new explorations of FLASH therapy, highly conformal SBRT, and microbeam therapy. With future work aimed towards prototyping the device, creating dose calculation, dose painting, and inverse planning software, designing on-board imaging for precise target localization, and developing proper QA protocols, this platform has significant potential to surpass the current state-of-the-art in small animal radiotherapy research.
