DETERMINATION OF THE SPOT SIZE OF A LINEAR ACCELERATOR FOR THE ACUROS XB DOSE CALCULATION ALGORITHM USING SMALL FIELDS

Due to new advances in radiotherapy treatments, such as the Volumetric Arc Modulation Technique (VMAT), the tools used to describe and represent the linear accelerator (LA) have been improved with a greater concern for dose calculation in small field situations. The AcurosXB algorithm presents interesting results for such problems, however, for the precise calculation is essential to choose the appropriate value of a parameter called Spot Size, which is a beam model parameter, being the representation of the primary source that models the Bremsstrahlung photons created in the target. The study aims to evaluate the influence of the SpotSize in the small field dose calculation using the AcurosXB dose calculation algorithm, seeking to understand the importance of the variation of the source size for the accuracy of the dose distribution calculation and to find its ideal value. Three square format models of Spot Size, 0.5mm² 1.0 mm² and 1.5 mm², through the Beam Model tool of TPS were developed, and evaluated in different field sizes and calculation grid sizes for the Varian TrueBeam STx Linear Accelerator (Millennium HD 120). The evaluation consists of verifying the absolute dose agreement on the central axis of the beam and the agreement of the planned and measured dose 2D plane through the Omni Pro I’mRT® Software (IBA), using Radiochromic Gafchromic®EBT3 films. For absolute dose measurements on the central axis with irradiation fields of 0.5cm2 and 1.0cm², the spot sizes of 0.5mm² and 1.0mm² showed the smallest percentage deviations (2.3% and 2.5%), respectively. Moreover, analyzing the dose profile, the 0.5mm² spot size model presented the best correlation results, given by the correlation index r, with values of 0.9918 and 0.9978, for the 0.5cm² and 1.0cm² fields, respectively. The 1.0mm² spot size model, which is the manufacturer defined standard spot size model, showed the best results for Gamma Index pass rates for the 1% 1mm criterion in both field sizes (0.5 cm2 and 1.0cm²), with approval rates of 99.9% and 95.6%, respectively. Thus, the results of this work show that the 0.5mm² and 1.0mm² square format spot size models presented the best results when comparing the irradiated measurements with the planned measurements, being these the most recommended models for the spot size configuration for the AcurosXB dose calculation algorithm.

Publication History

Article published online:
23 October 2019