Assessment of the Effect of Children's Bone Density Development in Pediatric MR Only Treatment Planning

MR only treatment planning for pediatric radiation therapy is helpful to reduce the patient dose and more precise target definition. Bone segmentation and assigning a suitable bulk electron density to bone tissue is important in this technique. Bone in children under 14 years old is still developing so the mineral density is changing during these ages. The objective of this study is to assess the effect of assigning the same bulk electron density to bone tissues of the children with different ages on dose distribution. Seven sets of skull CT images of children under 19 years old were selected. Skull bones were segmented and the CT numbers extracted, then the CT numbers converted to density. In order to compare the differences of dose distribution due to differences in bone density, the percentage depth dose was calculated by Monte Carlo simulation in inhomogeneous phantoms. The results of PDDs in photon and electron sources did not show a significant difference (<2%) between different densities beneath the bone tissue. When MR only treatment planning is to be used for a child, the bulk density method is accurate enough for treatment of brain or underneath area of bone. However, if the target of radiation therapy is bone, this method may cause a little error in dose calculation especially in superficial and electron therapy, so that voxel based methods are more reliable for these treatments.