A virtual-accelerator-based verification of a Monte Carlo dose calculation algorithm for electron beam treatment planning in clinical situations

Abstract

Background and purpose The introduction of Monte Carlo (MC) techniques for treatment planning and also for verification purposes will have considerable impact on the radiation therapy planning process. The aim of this work was to use a virtual accelerator to study the performance of a MC-based electron dose calculation algorithm, implemented in a commercial treatment planning system. Methods The performance in phantoms containing air and bone as well as in patient-specific geometries (thorax wall, nose, parotid gland and spinal cord) has been studied. Results The agreement between the virtual accelerator and the MC dose calculation algorithm is generally very good. A γ-evaluation with criteria of 0.03 Gy/3 mm (per Gy at the depth of maximum dose) shows that, even for the worst cases, only a small volume of about 1.5% has γ > 1.0. In the worst case, with the 0.02 Gy/2 mm criteria, about 92% of the volume receiving more than 0.85 Gy per 100 monitor units (MU) has γ-values <1.0. The corresponding value for the volume receiving more than 0.10 Gy/100 MU is about 98%. For the 18 MeV spinal-cord case, where a 6 × 20 cm 2 insert is used, the TPS underestimates the dose outside the primary field due to inadequate modelling of the insert. Conclusion The possibility of dose calculations in typical patient cases makes the virtual accelerator a powerful tool for validation and evaluation of dose calculation algorithms present in treatment planning systems.