Room: AAPM ePoster Library
In radiation therapy, it is important to accurately evaluate organ doses to predict potential side effects and minimize toxicities. However, computed tomography (CT) images used in treatment planning typically only cover a portion of the patient body near the target volume, so the non-target dose of out-of-field organs cannot be assessed. In addition, due to the limited CT resolution or incomplete contouring, lymphnodes or other small structures are often not considered. The aim of this study is to develop a method for organ dose assessment using a whole-body computational human phantom that is not limited by CT voxel resolution and Monte Carlo (MC) simulation using TOPAS (TOol for PArticle Simulation).
The TOPAS MC simulation toolkit was used to simulate therapeutic proton and photon beams for craniospinal irradiation of medulloblastoma patients. Non-voxelized, tetrahedral mesh-type reference computational human phantoms (MRCPs) were implemented in TOPAS and treatments were planned in a commercial planning system (TPS) based on voxelized representations of the phantoms. The average dose to the target volume and lymphnode was compared between TPS and TOPAS.
The dose difference to the target volume between TPS and TOPAS was within 4%. However, the dose to the lymphnodes calculated using MRCPs shows a significant difference from the TPS. For proton and photon treatments, the lymphnodes in the extrathoracic region and trunk region of MRCPs was 8 times and 2 times higher than predicted by TPS, respectively.
Our results shows the difficulty to accurately assess organ doses to small structures such as lymphnodes with commercial planning systems and voxelized structures.