Computer-aided segmentation on MRI for prostate radiotherapy, Part I: Quantifying human interobserver variability of the prostate and organs at risk and its impact on radiation dosimetry

•High interobserver variability (IoV) was observed at junctions between the prostate and one or more organs at risk.•Radiation oncologists were the most consistent group of observers, but high IoV was still observed in this group.•IoV demonstrated a dependence on organ size, and was highest for the external urinary sphincter.•High IoV was observed on both treatment planning MRIs and postimplant quality assessment MRIs.•Dose-volume-histogram parameters for MRI-based prostate radiotherapy are heavily influenced by IoV.