Monte Carlo modelling of a 6 MV Elekta linear accelerator for in-field and out-of-field dosimetry

In this study, a 6 MV Elekta linear accelerator was modelled and validated for in-field dosimetry against measurements by comparing the output factors, percentage depth dose, and lateral profiles. The resultant lateral profiles for the validated model were then compared with measurements for the out-of-field region up to 60 cm off-axis. It was initially found that there were discrepancies between the calculated and the measured profiles in the out-of-field region from 10 to 60 cm off-axis. Hence, two main adjustments were made to the length of jaws and materials of some of the head components in the LINAC model so as to be exactly as described by the manufacturer in order to acquire the best possible agreement with the measured values. Also, small head components such as the ion chamber, mirror, and Mylar sheet are included in the model to improve the results. Only in-line components were included in the model; the surrounding shielding components were excluded. The results after modifications of the head model showed good agreement with measurements up to 40 cm of field; however, from 40 to 60 cm there are still discrepancies of up to 80%, and the model underestimates the measured doses. Background radiation was measured inside the LINAC room and added as an error bar to measurements, which reduce the difference to 15% at distal points. This could possibly be due to the fact room scatter was not included in the model.