Abstract
Objective: The Monte Carlo (MC) technique can accurately model any linear accelerator where accurate details about the treatment head and the incident electron beam have been provided. However, manufacturers generally do not provide such details as the energy, radial intensity (spot size), or the angular spread of the incident electron beam. The aim of this study is to predict these details and validate a MC Linac model with the measurements.
Methodology: A 10 MV photon beam from an Elekta Synergy Linac was modelled using the BEAMnrc code. The percentage depth dose of the MC model was generated using different electron energies and compared with the measurements using a Gamma index (γ) with two different criteria sets. The dose profiles of the fine-tuned electron energy were generated using different spot sizes and compared with appropriate measurements. In addition, the fine-tuned electron energy and spot size dose profiles were generated using different angular spreads to minimise any differences. Finally, the output factors for different field sizes, with and without a wedge, and also the quality index, were compared.
Results: The fine-tuned electron energy of the MC model was found to be 9.8 MeV, where 94.12% of the calculation points pass the γ test. For the spot size, a circular radial intensity of 0.35 cm best described the 10 MV photon beam. Furthermore, a mean angular spread of 0.05° minimised the cross-field profile differences between calculation and measurement. The largest differences between the output factors from the MC model and measurements were -0.8% and 4.7% for an open and wedged field, respectively. Ultimately, a difference of 0.82% in the quality index was achieved.
Conclusion: A reliable MC model of a 10 MV photon beam Elekta Synergy Linac can be achieved as presented in this study using the BEAMnrc code. This model can be reliably used to calculate dose distributions.