In preclinical studies, the absorbed dose calculation accuracy in small animalsis fundamental to reliably investigate and understand observed biologicaleffects. This work investigated the use of the split exponential track lengthestimator (seTLE), a new kerma based Monte Carlo dose calculation methodfor preclinical radiotherapy using a small animal precision micro irradiator,the X-RAD 225Cx.Monte Carlo modelling of the irradiator with GATE/GEANT4 wasextensively evaluated by comparing measurements and simulations for halfvaluelayer, percent depth dose, off-axis profiles and output factors in waterand water-equivalent material for seven circular fields, from 20 mm down to1 mm in diameter. Simulated and measured dose distributions in cylinders ofwater obtained for a 360° arc were also compared using dose, distance-toagreementand gamma-index maps.Simulations and measurements agreed within 3% for all static beamconfigurations, with uncertainties estimated to 1% for the simulation and 3% forthe measurements. Distance-to-agreement accuracy was better to 0.14 mm. Forthe arc irradiations, gamma-index maps of 2D dose distributions showed thatthe success rate was higher than 98%, except for the 0.1 cm collimator (92%).Using the seTLE method, MC simulations compute 3D dose distributionswithin minutes for realistic beam configurations with a clinically acceptableaccuracy for beam diameter as small as 1 mm.