Document Type : Conference Proceedings
Medical Imaging Research Center, Shiraz University of Medical Sciences, Shiraz, Iran
Ionizing and Non-Ionizing Radiation Protection Research Center, Shiraz University of Medical Sciences, Shiraz, Iran Radiotherapy and Oncology Department, Shiraz University of Medical Sciences, Shiraz, Iran
Department of Medical Radiation Engineering, School of Mechanical Engineering, Shiraz University, Shiraz, Iran Radiation Research Center, School of Mechanical Engineering, Shiraz University, Shiraz, Iran
Introduction: MRI-only treatment planning (TP) can be advantageous in paediatric radiotherapy. However, electron density extraction is necessary for dose calculation. Normally, after bone segmentation, a bulk density is assigned. However, the variation of bone bulk density in patients makes the creation of pseudo CTs challenging. This study aims to assess the eﬀects of bone density variations in children on radiation attenuation and dose calculation for MRI-only TP.
Materials and Methods: Bone contents of<15-year-old children were calculated, and substituted in the Oak Ridge National Laboratory paediatric phantoms. The percentage depth dose and beam profile of 150 kVp and 6 MV photon and 6 MeV electron beams were then calculated using Xcom, MCNPX (Monte Carlo N-particle version X) and ORLN phantoms.
Results: Using 150 kVp X-rays, the diﬀerence in attenuation coefficient was almost 5% between an 11-year-old child and a newborn, and ~8% between an adult and a newborn. With megavoltage radiation, the diﬀerences were smaller but still important. For an 18 MV photon beam, the diﬀerence of radiation attenuation between an 11-year-old child and a newborn was 4% and~7.4% between an adult and a newborn. For 6 MeV electrons, dose diﬀerences were observed up to the 2 cm depth. The percentage depth dose diﬀerence between 1 and 10-year-olds was 18.5%, and between 10 and 15-year-olds was 24%.
Conclusion: The results suggest that for MRI-only TP of photon- or electron-beam radiotherapy, the bone densities of each age group should be defined separately for accurate dose calculation.