Influence of variance reduction techniques on conventional radiotherapy simulations with TOPAS MC

This study aims to apply Geometrical Particle Splitting (GPS) and Importance Sampling (Ant Colony Method) as variance reduction techniques (VRT) to improve computational efficiency in Monte Carlo calculations of the conventional radiotherapy beam quality index, TPR_20,10.

The TOPAS was used to simulate a setup to determine the PDD_20,10 ratio using a water phantom at a source-to-surface distance of 100 cm and a 10 × 10 cm² field. TPR_20,10 values were calculated for each simulation using the 6 MV Elekta Precise linear accelerator IAEA phase space as source. Control simulations without VRTs provided a baseline. The influence of the number of split planes was evaluated for both techniques. The effects of VRT on photons and electrons were evaluated, focusing on computational efficiency compared to simulations without VRT. A Z-test assessed bias by checking compatibility between simulated and experimental TPR_20,10 values.

TPR_20,10 results were validated for 3 σ compatibility with experimental data. The GPS technique's best results showed an efficiency gain factor of 4.01. Applying the technique to only photons or both photons and electrons did not yield significant differences. In Importance Sampling, the best results achieved a gain factor of 16.91 for both photons and electrons, but only yielded 0.75 for electrons alone.

VRTs significantly improved computational efficiency, though their effectiveness depends on geometry and particle selection. When applied carefully, these techniques enhance precision without greatly increasing processing time.