Abstract:

Objective To investigate the impact of statistical uncertainty per control point （SUpCP） on dose calculation on volumetric modulated arc therapy（VMAT） for rectum cancer， and to analyze the accuracy and efficiency of calculation. Methods 19 patients with rectum cancer undergoing radiotherapy were selected. The initial VMAT plans were generated on Monaco TPS using SUpCP = 3， then changed SUpCP in the dose calculation process as follow： 10 SUpCPs （1 ~ 10） for each patient， and totally190 VMAT dose distributions were obtained. For plan evaluation， D_max， D_mean， D_95%， V₅₀， homogeneity index（HI）， conformity index（CI） of the planning target volume（PTV）， dissymmetric variations of bladder， small intestine and femoral head， and time calculation（Time）were analyzed. Patient specific quality assurance（PSQA）， dose deviation of isocenter （ΔD_ISO）and passing rate of three-dimensional dose distribution（γ₃₃， γ₃₂， γ₂₂） between calculated and delivered radiation doses were measured. Results AsSUpC increased， D_max and HI of PTV， D_max of bladder were increased， but D_95% and V₅₀ of PTV， Time， γ₃₂ and γ₂₂ were decreased（P < 0.05）. D_max and CI of PTV， Dmean of bladder， Dmax and Dmean of small intestine and femoral head， ΔD_ISO and γ₃₃ showed no statistical significance （P > 0.05）. When ΔD_ISO < 1%， gamma passing rate > 90% for all VMAT plan. When SUpCP < 6， Dmax of PTV < 110% of the prescribed dose was obtained； while SUpCP > 2， time for dose calculation was less than 5 min. Conclusion For VMAT plan of rectum cancer on Monaco TPS using XVMC algorithm， 3% ~ 5% of statistical uncertainty per control point for dose calculation， and 3% 2 mm or 2% 2 mm gamma criteria for three-dimensional dose verification is recommended. This study provides clinical application basis for precise dose calculation of VMAT plan of rectum cancer.

Key words: statistical uncertainty, dose calculation, VMAT, rectum cancer