Impact of contouring variability on dose-volume metrics used in treatment plan optimization of prostate IMRT

Abstract

Background and Purpose: Contouring variability remains a major source of uncertainty in radiotherapy treatment planning. The objective of this study was to identify the effect of contouring variability on dose-volume histogram (DVH) metrics used for treatment plan optimization of prostate IMRT. Methods: A total of 25 observers were recruited to delineate the bladder, prostate, and rectum in a CT scan of low-risk prostate cancer. Dice similarity coefficients (DSC) were calculated between observer and an algorithmically-generated consensus contour. The observer contours were used to generate treatment plans and calculate DVH for each organ. The variance between DVH curves was calculated for the values D95% for prostate, and V65, 70, 75 Gy for bladder and rectum. Results: DSC for the bladder, prostate, and rectum were 0.971 ± 0.007, 0.838 ± 0.067, 0.771 ± 0.124, respectively. DVH variance for all three structures was primarily driven by differences in prostate contouring. Variations in rectal contouring had important additional impacts only on rectal DVH. Bladder contouring variation had little impact on DVH metrics. Conclusions: Although the rectum was the most inconsistently contoured structure, its variability did not impact DVH as much as prostate variability. It has been demonstrated that the dosimetric impact of contouring variability cannot be predicted solely with DSC.