Use of Monte Carlo techniques in external beam therapy of lung cancer

The dose calculation models implemented in clinical radiation therapy has increased gradually in the last few decades. Monte Carlo (MC) simulations of the radiotherapy dose delivery has the highest dose calculation accuracy. Implementation of MC techniques to the intensity modulated radiation therapy (IMRT) of lung cancer was the main purpose of this thesis. Elekta Synergy MLCi linear accelerator was modelled using EGSnrc/BEAMnrc Monte Carlo code system in the thesis. Model verification was performed with ionization chamber measurements in water phantom. After model verification, dose distributions obtained from two different treatment planning systems (TPSs) and MC simulation in virtual slab phantoms that include tissue inhomogeneity were compared. Finally, dose distributions were compared in a real patient situation of IMRT of lung cancer. A good agreement was observed between ionization chamber measurements and MC calculations. Huge calculation errors in both virtual phantom and real patient situations were observed with TPS with pencil-beam calculation algorithm in comparison with MC calculations. Better results were obtained with TPS with collapse cone convolution/superposition calculation algorithm in both cases. In conclusion, the MC model of the linac produced in this study can be used for both radiotherapy plan verification and dose calculation in the situations of small fields and tissue inhomogenities where TPSs fail.