In intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) static collimator angles are used per field or arc. While dynamic collimator rotation during beam on was investigated for C-arm linear accelerators, this was not investigated for O-ring ETHOS/Halcyon treatment systems. This work aims in the development of an optimization framework for treatment plans with dynamic collimator rotation during beam on for ETHOS/Halcyon systems. A previously developed in-house hybrid direct aperture optimization algorithm was first extended with a Monte Carlo (MC) model of the ETHOS/Halcyon treatment head including the double stack multi leaf collimator to calculate MC beamlet dose distributions and second with options for dynamic collimator rotation optimization for IMRT and VMAT. The framework was tested for a clinically motivated case showing improved dose sparing for organs at risk using dynamic collimator angle rotation compared with static collimator angle treatment plans, while the target coverage was the same
