Effect of statistical uncertainty on Monaco Monte-Carlo dose calculation

Purpose: The aim of this study is to evaluate influence of statistical uncertainty on Monte-Carlo dose calculation of Monaco 5.11 treatment planning system (TPS). Methods: Phantom with contoured C-Shape structure set was downloaded from AAPM website provided with TG119 report. VMAT plan was created for C-Shape test case using Monaco TPS for 6 MV Elekta Versa-HD linear-accelerator. Dose prescription and constraints were as per TG119. After optimizations, C-Shape plan was calculated with different statistical-uncertainty (i) 0.5%, 1.0%, 3.0% and 5.0% per control point and (ii) 0.5%, 1.0%, 3.0% and 5.0% per calculation. Base plan was calculated with 0.5% per control point. Results: Variations in PTV doses for different statistical-uncertainties with respect to 0.5% per control point were within PTV-D95: 82 cGy(1.64%); PTV-D10: 14.8 cGy(0.28%); Core-D10: 3.7 cGy(0.15%). MU required to deliver a plan (920 MU) were observed same with different statistical-uncertainty. Calculation time increases with decrease in statistical-uncertainty due to more number of histories. 2D-Gamma pass rate was ranging from 98.1% to 98.9% for analyzed statistical-uncertainties. Statistical-uncertainty 0.5% per control point showed higher Gamma pass-rate (98.9%). Conclusion: Minor variation (<1.64%) in dose volume parameters was observed with different statistical-uncertainties, whereas Monitor unit remain same. 3.0% per control point and 0.5% per calculation resulted in almost similar results and found optimal with reasonable calculation time in terms of plan quality and delivery accuracy (gamma pass-rate).