Background: Magnetic resonance image-guided radiation therapy (MR-IGRT) promises more precise and effective radiation treatments compared to conventional IGRT by using real-time on-board MR imaging. Under the influenc...Background: Magnetic resonance image-guided radiation therapy (MR-IGRT) promises more precise and effective radiation treatments compared to conventional IGRT by using real-time on-board MR imaging. Under the influence of a magnetic field, however, secondary electrons exiting a surface can be forced in a circular path and re-enter the medium, resulting in dose increase at a beam-exit surface, called the electron return effect (ERE). The purpose of the study is to compare the exit skin dose computed by Monte Carlo dose calculation with measurements using an adult anthropomorphic phantom and to measure the effect of skin dose reduction by adding 1 cm-thick bolus. Method: The plan was compared with measurements using an adult anthropomorphic phantom combined with radiochromic films and thermoluminescent dosimeters. We also measured the skin dose reduction by adding 1 cm-thick bolus on the frontal surface of the phantom. Results: We found that 1 cm-thick bolus reduced the skin dose by up to 20% both in measurements and calculations. The plan was found to overestimate the measured skin dose by about 10% and there was no significant difference in the bolus effect between the breast skin and the skin (without breast attachment) doses. Conclusion: In conclusion, we confirmed the ERE effect on the anthropomorphic phantom under the magnetic field and the exit skin dose reduction by adding a bolus. Skin dose measurements using anthropomorphic phantom may be helpful to evaluate more realistic skin dose and the bolus effect in the magnetic field.展开更多
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 o...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.展开更多
文摘Background: Magnetic resonance image-guided radiation therapy (MR-IGRT) promises more precise and effective radiation treatments compared to conventional IGRT by using real-time on-board MR imaging. Under the influence of a magnetic field, however, secondary electrons exiting a surface can be forced in a circular path and re-enter the medium, resulting in dose increase at a beam-exit surface, called the electron return effect (ERE). The purpose of the study is to compare the exit skin dose computed by Monte Carlo dose calculation with measurements using an adult anthropomorphic phantom and to measure the effect of skin dose reduction by adding 1 cm-thick bolus. Method: The plan was compared with measurements using an adult anthropomorphic phantom combined with radiochromic films and thermoluminescent dosimeters. We also measured the skin dose reduction by adding 1 cm-thick bolus on the frontal surface of the phantom. Results: We found that 1 cm-thick bolus reduced the skin dose by up to 20% both in measurements and calculations. The plan was found to overestimate the measured skin dose by about 10% and there was no significant difference in the bolus effect between the breast skin and the skin (without breast attachment) doses. Conclusion: In conclusion, we confirmed the ERE effect on the anthropomorphic phantom under the magnetic field and the exit skin dose reduction by adding a bolus. Skin dose measurements using anthropomorphic phantom may be helpful to evaluate more realistic skin dose and the bolus effect in the magnetic field.
文摘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.
