Assmentment of Dose Simulation Uncertainties of a Pencil-Beam Based Treatment Planning System for the Chang Gung Memorial Hospital Proton Therapy Beam Lines

In July 2008, the Chang Gung Memorial Hospital (CGMH) signed a contract with Sumitomo Heavy Industry, Japan to start the project of building a state-of-the-art proton facility. The proposed beam modification methods for the gantries 1 & 2 of the CGMH system combines the wobbling technique for lateral spreading and the layer-stacking technique for forming a longitudinal spread-out Bragg peak (SOBP). This project has chosen the Eclipse treatment planning system as the primary treatment planning system (TPS). Its proton dose model assumes that a change in the nominal beam energy will change the peak to plateau ratio of the Bragg peak and its shape and a change in nozzle-equivalent-thickness (WeT) of all absorbing material inside the nozzle preserves the shape of the depth dose curve, but shifts its penetration in the patient. The assumptions may cause dose errors because it ignores the multiple Coulomb scattering process within the absorbing material which may results in overestimate of the beam penetration depths. Exactly how severe this dose error will be is a subject of intensive studies. The purpose of the current grant proposal is to explore the dose uncertainties caused by this assumption. The GEANT4 Monte Carlo package is proposed as the tool to study the possible dose uncertainty. Bragg peaks with and without the simplification made by the Eclipse TPS will be simulated and compared to explore their difference. Five physical parameters which characterize a Bragg peak will be used as the evaluation indices. They are (a) depth of penetration defined by the 90% dose (d90%), (b) depth of peak (dpeak), (c) width of 50% doses (W50%), (d) peak-to-entrance dose ration (Rpeak/ent), and (e) width of 80-20% distal dose falloff (W80-20%). The study will result in plots of parameter X vs. WeT and parameter X vs. nominal energy for all the five evaluation indices. These plots can serves as reference data during decision-making process to determine if additional measurement should be taken to reduce the dose calculation uncertainty of the TPS system.

Project ID:PC10108-0924
External Project ID:NSC101-2314-B182-068