Experimental parameterization of pencil beam data for energy scanning in heavy-ion therapy

Purpose: Three-dimensional (3D) pencil-beam scanning technique has been utilized since 2011 at the Heavy Ion Medical Accelerator in Chiba (HIMAC). At present, for depth direction, the hybrid depth scanning method has been employed, in which 11 beam energies ranging from 140 to 430 MeV/u are used in conjunction with the range shifter. To suppress the beam spread due the multiple scattering and the nuclear reaction, we have developed a multi-energy (more than 200 energy steps) scanning method (ES). To calculate the dose distribution accurately, it is necessary to obtain the pencil beam dataset including the contribution due to the large-angle scattered (LAS) particles for ES. However, the measurements or Monte Carlo (MC) calculations to derive all parameters are too time consuming. In addition, the dataset to realize dose optimization with the triple-Gaussian (TG) model is required. In this presentation, we propose a new approach to obtain the pencil beam dataset for the TG model in a short period of time.Methods: To derive the parameters describing the LAS particles and the integral depth dose (IDD), we measured the pencil beam dose distribution in water with a parallel-plate ionization chamber with concentric electrodes (PPIC with CE). Since the sensitive volume of each channel is increased linearly with radial distance, it is possible to efficiently and quickly detect small contributions from the LAS particles. To reduce time spent on beam data preparation, we selected 26 energy steps and others were derived by an interpolation method. Under the assumption that the first component was dominated by the primary ions, the lateral spread of the first component was calculated analytically. The fraction of the first component was derived by adjusting to obtain better agreements between measured results and calculations with the GEANT4. The second and third components were derived from the measured results. Here, the lateral spreads of the second and third components were approximated as invariant with respect to the depth. To evaluate the validity of this method, the new pencil beam dataset was applied to the dose calculation in the planning software.Results: The scaling factor to compensate for the dose reduction calculated with new parameters was compared with the existing one. The difference between the new scaling factor and the existing one was about 1%. The calculated dose distribution with the new parameters was in good agreement with the measured one.Conclusions: This method makes it possible to easily obtain the pencil beam dataset including the contribution due to the LAS particles, while maintaining the dose calculation accuracy. In addition, owing to both PPIC with CE and this method, the beam time was shortened to about 1/20 of the one without this method.53rd Annual Conference of the Particle Therapy Co-Operative Group (PTCOG53