Monte Carlo Simulation of a Novel GRID Therapy LINAC Treatment Head Design with Pencil Beam Scanning: a Feasibility Study

Abstract

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 tungsten X-ray target (0.18 cm thickness) and two stationary tungsten grid collimators. All collimators had parallel 2 cm center-to-center (ctc) distance square grids. The magnetic field strengths for dipole magnets were determined through derivation, and the collimator pairs parameters were determined through Bayesian optimization. A cubic water phantom (15×15×15 cm^3) was placed with d_max at isocenter with a source-to-axis distance (SAD) equals 75 cm and collimator-to-axis distance (CAD) equals 40 cm. The nominal energy of 6.51 MeV electron source was selected to represent the standard 6 MV LINAC output energy. The grid collimators were verified by comparing dose profiles between a single collimated 1 cm Full-Width-at-Half-Maximum beamlet with the open field photon, both percent depth dose (PDD) were determined. GRID profiles are verified through multi beamlets results to determine peak-to-valley dose ratio (PVDR) and the equivalent uniformed dose (EUD). Dose rate is estimated using simulated dose efficiency (in Gy/C) with 0.1 mA mean beam current. Results: The optimized dose rate was 2278 cGy/min with an output factor of 0.850 for a 1 cm FWHM beamlet. The GRID profile showed PVDR=18.14. Conclusions: This study proposed a novel LINAC treatment head design with scanning beam and grid collimator for delivering 1 cm FWHM photon and electron beamlets. This compact and dose efficient design may potentially provide a better platform for GRID therapy studies.