TU-EE-A2-04: A Hybrid Dose Evaluation Method for Rapid Monte Carlo-Based IMRT Optimization

Abstract

Purpose: To develop and test a hybrid method for optimizing IMRT plans using Monte Carlo(MC) dose calculation algorithms which retains the accuracy of MC while reducing the MC dose computational requirements. Method and Materials: A hybrid dose calculation strategy in which initial optimization is performed with a fast pencil beam (PB) algorithm using deliverable‐based IMRToptimization. Following convergence, doses are re‐computed with the VMC++ MC algorithm to determine correction factors for further PB‐based optimization. The correction/re‐optimization procedure is repeated until convergence. The hybrid method was benchmarked with respect to MC‐deliverable—based optimization for 5‐prostate IMRT plans. Figures of merit included number of MC dose computations required, final plan quality score, and optimization dose‐volume indices. Results: The hybrid method required a maximum of 3 MC dose calculations to converge to a result which provided equivalent dose coverage to the complete MC‐based optimization plan. The complete MC‐based optimization required between 6 and 9 MC dose computations to converge, depending on the specific patient. After 2 MC dose computations, the hybrid plan quality score was equal to or less than the MC‐based score for 4 of the 5 plans, the remaining plan required 3 iterations to achieve a score equal to that for the MC‐based optimization. Monitor units for the hybrid and complete MC‐optimization were within 5%. Conclusion:Hybrid PB‐MC‐IMRT dose calculation method is practical and results in plans equivalent to those achieved when MC‐dose calculation is used for all optimization iterations. The hybrid method reduced the number of MC calculations by a factor of ∼3, reducing overall optimization time by a factor of 2.8, and allowing for VMC++ MC‐based optimization to be completed in <30 min on a 20×2.4 Ghz CPU cluster. (Supported in part by NIH‐R01CA98524).