부산대학교 도서관

Objective: To investigate the impact of different magnetic field strengths and orientations on the dose deposition from lateral opposing photon beams irradiating a laryngeal geometry, with particular attention focused on the dose homogeneity of the area around the air cavity inside the larynx. Methods: Geant4 simulation toolkit was used to perform Monte Carlo simulations on a phantom resembling the larynx. The energy spectrum of a 6 ​MV photon beam from a Varian Clinac iX machine was used to provide the source of radiation. Three configurations of the Magnetic Resonance Image-Guided Radiation Therapy (MRIgRT) system were simulated: the Fixed Cylindrical (FC) geometry, the Longitudinal Rotating Biplanar (LRBP) geometry, and the Transverse Rotating Biplanar (TRBP) geometry. Uniform magnetic fields of different field strengths were applied. The relative dose maps and the homogeneity index (HI) were obtained for evaluations. Results: The LRBP geometry together with a magnetic field strength of 0.5 ​T produced the most homogeneous dose distribution with a HI of 0.139 for the region of interest around the air cavity, compared with a HI of 0.180 when the magnetic field was absent. For TRBP geometry, the dose distribution shifted towards the cranial direction, leading to a region of hot spots at the superior edge and a region of cold spots at the inferior edge of the irradiated volume. The HI around the air cavity was therefore worse. In addition, strong electron streaming effect (ESE) was observed. For the FC geometry, hot and cold spots were formed around the air cavity which increased the HI. Conclusions: Out of the three configurations of the MRIgRT systems, the LRBP geometry produced the most clinically beneficial dose distribution in which it provided a higher dose to the treatment area and a boost in dose to the tissue immediately adjacent to the air cavity.