Pilot study: rectal contouring variability in patients treated with radical radiotherapy for prostate cancer: impact on rectum dose-volume histograms

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 cm(3) (+/- 22.3 cm(3)). 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.