Influence of organ at risk definition on rectal dose-volume histograms in patients with prostate cancer undergoing external-beam radiotherapy

Purpose: To evaluate rectal close-volume relations during three-dimensional conformal radiotherapy of patients with prostate cancer by means of different rectal volume contours. Patients and Methods: 55 patients with prostate cancer underwent three-dimensional conformal external-beam radiotherapy. Rectal dose-volume histograms were calculated for four separately contoured rectal volumes in all patients resulting in four groups. In group 1 the outer rectal wall was contoured two CT slices above and below the planning target volume. The rectal contour of group 2 was drawn from the anal verge up to the sigmoid. Furthermore, the posterior half of the rectum was contoured for both volumes mentioned above (groups 1a and 2a). Statistical analysis was then performed using nonparametric WiLcoxon tests. Results: The mean target dose was 72.9 Gy (standard deviation [SD] +/- 2.1 Gy). The minimum target dose was 70.2 Gy. Mean rectum dose ( SD) over all patients was 50.7 Gy (+/- 4.6 Gy), 45.2 Gy (+/- 5.4 Gy), 43.2 Gy (+/- 4.2 Gy), and 38.7 Gy (+/- 5.5 Gy) for group 1, 2, la, and 2a, respectively. The corresponding volumes receiving >= 70 Gy for groups 1 and 2 were 14.0% (+/- 5.3%) and 11.9% ( +/- 4.5%). These differences were statistically significant. Comparison of minimum and mean rectal dose also revealed a statistically significant difference toward higher doses in groups 1 and 1a (p < 0.001). Maximum rectal doses for groups 1 and 2 as well as for groups la and 2a revealed no statistically significant difference (p = 1.0). Conclusion: Data from the literature on normal-tissue complication probability (rectal bleeding) refer to different rectal contours. When applying dose restrictions to the rectum, contouring becomes a significant factor that determines the risk of rectal toxicity. The results of this study show that different ways of rectal contouring significantly influence doses to the rectum. The influence of organ at risk contouring should be considered thoroughly in conformal radiotherapy of prostate cancer patients, especially in dose escalation studies. It is recommended to calculate the doses for absolute rectal volumes and correlate these data with toxicity in order to be able to achieve comparable results among different institutions.