Experimental validation of Geant4 nuclear interaction models in dose calculations of therapeutic carbon ion beams

The choice of nuclear interaction models in Monte Carlo simulations affects the dose calculation accuracy for light ion beam therapy.

This study aimed to evaluate the dose calculation accuracy and simulation time of three GATE-RTiON/Geant4 physics lists for therapeutic carbon ion beams, assessing their suitability for independent dose calculation in patient-specific quality assurance (PSQA).

The normalized beam models for physics lists QGSP_BIC_HP_EMZ, QGSP_INCLXX_HP_EMZ, and Shielding_EMZ were validated against measurements regarding the accuracy of range, spot size and reference dose. Normalized transversal dose profiles ( $D/D_{max}$ ) and field size factor (FSF) were compared with measurements. The accuracy of simulated target dose in 103 fields (various energies, field sizes, depths, and dose gradient ${\nabla }_D$ complexity) of energy-modulated scanned beams was evaluated at 3181 positions. The median of global dose difference $med\left({\Delta }_D\right)$ was calculated at different depth ranges.

The three physics lists with validated beam models showed similar accuracy in $D/D_{max}$ and FSF in the Bragg peak region and proximal depths, while QGSP_INCLXX_HP agreed most closely for $D/D_{max}$ in the fragmentation tail. Accounting for ${\nabla }_D$ -related uncertainty, $med\left({\Delta }_D\right)$ remained within ±1.1% for QGSP_INCLXX_HP, while exhibiting an overall increasing trend with depth for QGSP_BIC_HP (up to 2.3%) and a decreasing trend for Shielding (down to -4.1%), respectively. By tuning the number-of-primaries/monitor unit conversion ( $k_{N/\mathrm{MU}}$ ) as a function of energy, $med\left({\Delta }_D\right)$ of QGSP_BIC_HP was reduced to within ±1.3%, at the cost of reduced accuracy in the simulated reference dose. The simulation time of Shielding was 1.8 times that of QGSP_BIC_HP and 1.5 times that of QGSP_INCLXX_HP.

QGSP_INCLXX_HP demonstrated high dosimetric accuracy in the target region of energy-modulated fields. QGSP_BIC_HP and Shielding showed physics model-related inaccuracies in simulated target dose. Additional $k_{N/\mathrm{MU}}$ tuning improved their target dose calculation accuracy with a trade-off of reference dose accuracy. The computationally efficient QGSP_INCLXX_HP and QGSP_BIC_HP are viable candidates for dose calculation applications of carbon ion beam therapy, such as in silico PSQA.