Effects of Statistical Fluctuation in Monte Carlo Photon Beam Dose Calculation On Gamma Index Evaluation
Y Graves*, X Jia, S Jiang, Center for Advanced Radiotherapy Technologies, University of California, San Diego, La Jolla, CASU-E-T-165 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To systematically investigate the impact of the Monte Carlo (MC) statistical fluctuation on the gamma-index test results.
Methods: Firstly, we build a simplified model that theoretically demonstrates, to the first order approximation, the MC noise tends to overestimate gamma-index values when existing in the reference dose and underestimate gamma-index values when existing in the evaluation dose given the original gamma-index is relatively large for the noise level. Secondly, we validate the theoretical conclusions using real clinical data: a IMRT prostate and a VMAT HN plan. We first conduct a base gamma-index comparison between the non-MC dose and the MC dose with zero noise level. We then compute average gamma-index value, when the non-MC dose is compared with the MC doses of different noise levels. Furthermore, the gamma passing rates are also reported for each comparison between the non-MC dose and MC doses.
Results: Our numerical experiments show that 1) when performing a gamma-index test between an MC reference dose and a non-MC evaluation dose, the average gamma-index is overestimated and the passing rate decreases with the increase of noise levels in the reference dose; 2) when performing the test between a non-MC reference dose and an MC evaluation dose, the average gamma-index is underestimated when they are within the clinically relevant range and the passing rate increases with the increase of noise levels in the evaluation dose; 3) when performing a gamma-index test between an MC reference dose and an MC evaluation dose, the passing rate is overestimated when the noise in the evaluation dose dominates, and underestimated when the noise in the reference dose dominates.
Conclusion: Gamma-index tests should be used with caution when comparing dose distributions computed with MC simulations.