Purpose: The organ at risk in prostate radiotherapy is the anterior rectal wall . This pilot study was conducted to quantify the inter-observer variability of rectal contouring in conformal prostate radiotherapy planning using four contouring methods and to determine a standard rectal contouring definition.
Methods and materials: Ten patients with T1/T2 disease and six clinical oncologists were recruited. Two cross-sectional and two length limits generated four rectal volumes. Each clinician contoured the four volumes for each patient and the dose–volume histograms (DVHs) were analysed. The percentage rectal volume receiving 20%, 50%, 80%, 90% and 95% of the total delivered dose and the mean and median rectal doses were calculated. Data were presented as mean ± 2 standard deviations.
Results: The Sh Rec method (contouring the rectum including its contents extending from 2 cm above the upper limit of the prostate to 2 cm below the prostatic apex) was the least variable in the 80%, 90% and 95% percentage ranges. The mean difference in Sh Rec-contoured volume was 18.7 cm3 (± 22.3 cm3).
Conclusions: The Sh Rec-contouring method showed the least inter-observer variability. The results are informative and will help define a standard rectal contouring method.