In principle, proton therapy offers a substantial clinical advantage over conventional photon therapy. This is due to a more favorable depth-dose distribution of protons, which allows greater sparing of normal tissues and improve local tumor control. On the other hand, proton therapy is more sensitive to movements and changes in the anatomy. Adding large safety margins in treatment planning dramatically reduces the benefits of proton therapy. The biggest movements happen in the thorax during breathing. Most proton therapy centers do not (yet) irradiate lung tumors, since treatment plans with current irradiation techniques are not significantly better and there are no clinical evidence supporting its benefits compared to conformal photon plans. In this master's thesis, the process of simulation of the predicted dose distribution is presented for a patient that breathes during the treatment. Two breathing phases were considered - the inhalation and the exhalation phase. The comulative dose distribution is evaluated in one (reference) respiratory phase. Dose distribution from the exhalation phase was transformed into the inhalation phase. The transformation was obtained in the process of image registration using the program Elastix and the dose distribution was simulated with MCNP Monte Carlo code.
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