Interobserver variability in rectum contouring in high-dose-rate brachytherapy for prostate cancer: A multi-institutional prospective analysis

Abstract Purpose The aim of this study was to evaluate the interobserver variability (IOV) of rectum contouring, and its dosimetric consequences, for high-dose-rate brachytherapy in patients with prostate cancer across multiple institutions. Methods and Materials Five radiation oncologists contoured rectums in 10 patients on transperineal ultrasound image sets after establishing a delineation consensus. The D 0.1cc , D 1cc , and D 2cc rectum volume parameters were determined. The mean, standard deviation, and range of each dose–volume histogram parameter were evaluated for each patient. The IOV was determined using the coefficient of variation, and the dosimetric impacts on the total dose were analyzed by estimating the biologically equivalent dose (EQD 2α/β = 3 ). Results The interobserver coefficients of variation (±standard deviation) for the reported D 0.1cc , D 1cc , and D 2cc were 5 ± 1.84%, 4 ± 1.26%, and 4 ± 1.33%, respectively. As for the impact on the total dose, the mean dose differences for D 0.1cc , D 1cc , and D 2cc were 10 Gy, 7.3 Gy, and 6.6 Gy, respectively. Conclusions The D 2cc is robust as evident by the low IOV (