When planning treatment using Radiation Treatment Planning system (RTPs), errors in the contouring process,
which is the pre-planning stage, reduce the accuracy of the treatment. Therefore, we wanted to evaluate
the error rate of RTPs, Window Width (WW), Window Level (WL), Hounsfield Unit (HU) values at the set
point, and the error rate by organ, which are the causes of errors in the contouring process. The results showed
that the error rate of WW, WL and HU of certain areas decreased over time. There were no significant changes
in the other variables. Therefore, when performing auto-contouring for each organ, appropriate WW, WL and
HU values should be set and additional manual contouring should be performed to ensure the accuracy of the
treatment plan.