Abstract: A collimator assembly for a linear accelerator for use in FLASH radiotherapy includes a collimator and a tray. The collimator includes an elongated body and a base and can also include an opening that defines a central bore formed in the elongated body and the base and extending the length of the collimator along a central axis. The tray includes a central opening having a larger cross-section than the elongated body of the collimator but smaller than the base of the collimator to allow only the elongated body of the collimator to pass therethrough such that the collimator is retained by the tray for reversible installation on the linear accelerator. The tray is adapted to be removably installed on accessory rails of the linear accelerator. Both the collimator and tray may be fabricated by 3D printing to allow for customization.