Author: Yiran Liu, Zhengzheng Liu, Fang-Fang Yin 👨🔬
Affiliation: Duke Kunshan Univeristy, Huazhong University of Science and Technology, Medical Physics Graduate Program, Duke Kunshan University 🌍
Purpose: To investigate the feasibility of producing high dose rate GRID therapy using a compact linear accelerator (LINAC) treatment head with scanning beam and stationary grid collimator.
Methods: A simulation study (TOPAS Monte Carlo toolkit, Version 3.9) was performed with a flattening-filter-free and multileaf-collimator-free treatment head. The orientation of electron beams was steered by two pairs of orthogonal dipole magnets and followed by additional beam-shaping systems for photon and electron modes, respectively. The photon mode consists of a tungsten target (0.18 cm thickness) and stationary tungsten grid collimator pairs: primary (4 cm thickness, 2 mm grid side length) and secondary (6 cm thickness, 5.6 mm grid side length). The electron mode consists of a thin brass grid collimator (0.48 cm thickness, 5 mm grid side length). All collimators had parallel 2 cm center-to-center distance square grids. A cubic water phantom (15 cm3) was placed in isocenter with the source-to-axis distance of 68 cm for the photon mode and 55.5 cm for the electron mode. The nominal energy of 6.51 MeV electron source was selected to represent the standard 6 MV LINAC output energy. The beam-shaping system was verified by comparing dose profiles between a collimated 1 cm Full-Width-at-Half-Maximum beamlet with an open field of 15×15 cm for photon and 2×2 cm for electron mode. Both dose rate (for 0.18 mA mean current), and peak-to-valley dose ratio (PVDR) are calculated for evaluation.
Results: The dose rate and PVDR are 667.4 cGy/min and 5.7, in the photon mode, and are 241.2 Gy/s and 2.6,in the electron mode.
Conclusion: This study proposed a novel LINAC treatment head design with scanning beam and grid collimator for delivering 1 cm Full-Width-at-Half-Maximum photon and electron beamlets. This compact and dose efficient design may potentially provide a better platform for GRID therapy studies.