A novel protocol for parametrization of a beam model for small stereotactic beams

Most Treatment Planning Systems (TPS) utilize parameterized multi-source models to represent the radiation beam of linacs. However, small stereotactic beams require special attention due to the importance of correct spot- and field-size definition. The purpose of this work is to develop a protocol for configuring a beam model that provides accurate representation of stereotactic beams.

6 MV flattening filter free (FFF) dose-profiles were measured for Millennium multileaf collimator (MLC)-collimated field sizes (2 × 2 cm²-5 × 5 cm²). They served as a reference against which corresponding TPS-profiles were optimized by configuring the Dosimetric Leaf Gap (DLG) and the Effective Spot Size (ESS). The optimized model was evaluated by comparing calculated and measured stereotactic radiotherapy (SRT)-plans. The proposed protocol was implemented on Anisotropic Analytical Algorithm (AAA) and Acuros XB in the Eclipse TPS.

Improvement in the agreement between measured and modelled profiles are identified using the suggested protocol. The DLG was determined at an optimum of 0.9 mm for both models whereas the ESS was determined to be (0.5, 0) mm and (1, 0,5) mm for AAA and Acuros XB, respectively. Compared to the standardly configured clinical model, the optimized AAA (Acuros XB) model showed an average improvement in gamma pass-rates of 2% (3%) with film measurements.

The standard protocol for TPS commissioning was shown to produce a sub-optimal beam model for small treatment volumes. The proposed protocol in this work results in a better modelling of small stereotactic beams.