Determination of the dose received by critical organs in external radiotherapy by monte carlo method

Radiotherapy plays a critical role in cancer treatment to provide the balance between targeting tumor tissues and protecting surrounding healthy tissues. In external radiotherapy, especially the use of high-energy Bremsstrahlung photons provides effective penetration into deep tissues, but may lead to undesirable dose accumulation in critical organs. This study aims to precisely determine the dose received by critical organs using the Monte Carlo method. Within the scope of the thesis, simulations were performed on a 3D model phantom for 6, 10 and 18 MV Bremsstrahlung photon sources using the Particle and Heavy Ion Transport Code System software. The phantom was designed based on the prostate cancer treatment scenario, and the gross tumor volume, clinical target volume and surrounding critical structures (bladder, bowels, pelvis) were modeled with a detailed geometry. In the simulations, photon flux distributions, center deviation rates, percentage depth doses and beam profiles of different energy levels were analyzed. The findings showed that dose penetration deepens as energy increases, but exposure in critical organs also varies depending on the energy. For example, while the surface dose is higher at 6 MV energy, a more homogeneous dose distribution is achieved in deep tissues with 18 MV. However, unexpected dose increases were observed in critical organs due to secondary electron contamination and lateral scattering at high energies. Beam flatness and symmetry analyses revealed that 6 and 10 MV energies exhibited balanced performance, especially in symmetry, while the accuracy and flexibility of the PHITS code for clinical dosimetry needed to be improved. This study emphasizes that Monte Carlo methods are a convenient tool for optimizing critical organ doses in external radiotherapy planning. The obtained results provide significant contributions to clinical practices in terms of personalizing treatment protocols and reducing radiation-induced side effects.