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

Purpose The aim of this study is to provide and test a new methodology to adjust the AcurosXB beam model for VMAT treatment plans. Method The effective target spot size of the AcurosXB v15 algorithm was adjusted in order to minimize the difference between calculated and measured penumbras. The dosimetric leaf gap (DLG) was adjusted using the asynchronous oscillating sweeping gap tests defined in the literature and the MLC transmission was measured. The impact of the four parameters on the small field output factors was assessed using a design of experiment methodology. Patient quality controls were performed for the three beam models investigated including two energies and two MLC models. Results Effective target spot sizes differed from the manufacturer recommendations and strongly depended on the MLC model considered. DLG values ranged from 0.7 to 2.3 mm and were found to be larger than the ones based on the sweeping gap tests. All parameters were found to significantly influence the calculated output factors, especially for the 0.5 cm × 0.5 cm field size. Interactions were also identified for fields smaller than 2 cm × 2 cm, suggesting that adjusting the parameters on the small field output factors should be done with caution. All patient quality controls passed the universal action limit of 90%. Conclusion The methodology provided is simple to implement in clinical practice. It was validated for three beam models covering a large variety of treatment types and localizations.

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