Room: AAPM ePoster Library
Purpose: Obtaining good agreement between Monte Carlo SRS calculations and commercial second check software can be difficult. At the U of R we perform second checks of these fields using AcurosXB to avoid this problem. To evaluate the agreement between the two algorithms for SRS/SBRT cases, we compared the dose calculated by Elements and Acuros to ion chamber (IC) measurements.
Methods: A homogeneous cylindrical phantom was scanned and imported into BrainLab for planning. The diameter (d) of the PTVs at the isocenter varied from 1 to 8cm. Treatment plans of 30Gy/5fxs for each PTV size were generated with BrainLab Elements Cranial SRS (v1.5.1), where each plan had identical gantry and table motions. The optimized VMAT plans were exported to Eclipse and recalculated with AcurosXB (v13.6). The IC measurement was performed by placing an Exradin A1SL at the isocenter or off-axis points along a vertical line in the phantom with distance (y) from the isocenter ranging from 5.5 to 13.5cm, so that only the scattered dose component contributes at the farthest point.
Results: 1) Isocenter: Elements and Acuros were lower than IC measurements by -0.7% and -1.6% on average, respectively. The difference between the two TPSs were 0.28% for a PTV(d=1cm) and 1.94% for a PTV(d=8cm). 2) Out-of-field: in general Elements was higher than IC, while Acuros was lower than IC. Elements showed slightly better agreement in overall. However, Elements had several potentially statistically noisy dose calculations that gave a bigger difference than Acuros; at 18% of the prescription dose for a (PTV(d=8cm)/y=13.5cm) the dose difference for Elements was 26.8cGy (4.9% from the IC dose), while the Acuros difference was 4.7cGy (0.9% from IC) at the same point.
Conclusion: Both Acuros and Elements calculations showed acceptable agreement with IC measurement, but these results show the potential for differences at the off-axis.