Discrepancies in dose distributions due to age- dependent bone electron density in pediatric MRI-only treatment planning: A Monte Carlo study

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<۱۵-year-old children were calculated, and substituted in the Oak Ridge National Laboratory paediatric phantoms. The percentage depth dose and beam profile of ۱۵۰ kVp and ۶ MV photon and ۶ MeV electron beams were then calculated using Xcom, MCNPX (Monte Carlo N-particle version X) and ORLN phantoms. Results: Using ۱۵۰ kVp X-rays, the difference in attenuation coefficient was almost ۵% between an ۱۱-year-old child and a newborn, and ~۸% between an adult and a newborn. With megavoltage radiation, the differences were smaller but still important. For an ۱۸ MV photon beam, the difference of radiation attenuation between an ۱۱-year-old child and a newborn was ۴% and~۷.۴% between an adult and a newborn. For ۶ MeV electrons, dose differences were observed up to the ۲ cm depth. The percentage depth dose difference between ۱ and ۱۰-year-olds was ۱۸.۵%, and between ۱۰ and ۱۵-year-olds was ۲۴%. 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.

کلیدواژه ها:

MR only treatment planning ، Electron density ، Pediatric radiotherapy ، Monte Carlo ، ORNL Phantom