Objective To explore the effect of the expansion of auto-contouring organ at risk (OAR) on various dose evaluation parameters.

Methods The stomach and duodenum were automatically and manually contoured on the CT scans of 50 individuals who had upper abdominal radiotherapy from 2019 to 2021. Mann-Whitney U test was used to assess the difference in auto-contouring results between stomach and duodenum in terms of Dice similarity coefficient (DSC) and 95% Hausdorff distance (HD95). To calculate the dose difference between the auto-contouring and manually contouring with the clinically applied dose distribution. The Wilcoxon rank-sum test was utilized to evaluate the variation of the dose evaluation parameters caused by the different expansion distance. The lower limit of the 95% confidence interval for each difference averages was calculated by bootstrap method. The minimum expansion distance required when the dose evaluation parameter corresponding to the expanded structure was greater than or equal to that for the manual contoured OAR was analyzed.

Results The DSC and HD95 of stomach were superior to those of the duodenum and there was a significant difference (P < 0.001). There were significant differences in the maximum doses induced by adjacent expansion procedures (P < 0.001). Under the condition of DSC > 0.8, the maximum dose of the expanded structure was larger than or equal to the manually contoured OAR when the outer expansion was around 7 mm; and the difference was not be greater than 1 Gy, even if it was less than the OAR that was manually contoured when the outer expansion was 5 mm.

Conclusions The maximum dose would change as the auto-contoured OAR expanding, while the mean dose and V₃₀ would not. A certain distance of expansion could ensure that the maximum dose corresponding to the expanded structure is greater than or equal to the maximum dose corresponding to the manually contoured OAR.