The impact of contouring uncertainty on rectal 3D dose-volume data: results of a dummy run in a multicenter trial (AIROPROS01-02).

PURPOSE: To estimate the impact of the uncertainty in contouring the rectum on rectal dose-volume parameters and normal tissue complication probability (NTCP) in a prospective (AIROPROS01-02) investigation about rectal toxicity. METHODS AND MATERIALS: The participants in a prospective trial (18 observers) were asked to draw the external contour of the rectum of 4 sample patients (3 patients undergoing radical conformal radiotherapy, 1 patient undergoing post-prostatectomy) on CT images (0.5 cm spacing) using a 3D treatment planning system. A previously accepted definition of cranial and caudal borders of the rectum was applied. For each patient, four- and six-field 3D-conformal techniques (70-76 Gy, ICRU dose) were planned and DVH/dose statistics of the rectum were calculated. The impact of interobserver variability on rectal volume, cranial and caudal borders, mean, maximum, and median rectal dose, percentage of rectum receiving more than 40, 45, 50, 55, 60, 65, 70, and 75 Gy (V(40)-V(75)), and NTCP were investigated. RESULTS: Concerning DVHs, 9/18 observers tended to have some systematic deviation. However, deviations from the mean values greater than 5% were found only in 1/9 because of a systematic discrepancy in the caudal limit assessment (mean deviation from the most frequently chosen slice: 8 mm). No other observers showed a mean deviation in the cranial or the caudal limit definition greater than 5.8 mm. For another observer, it was possible to clearly assess the cause of a relatively large systematic deviation for DVH parameters. In both cases, the observers were contacted to avoid these systematic deviations. When considering the remaining 16/18 observers, the average values of SD for V(40)-V(75) ranged between 1% and 4% and were found to be lower (<3%) for the 3 nonoperated patients. The average values of the SD were around 1.5-2 Gy and less than 1.5% for mean/median dose and NTCPs, respectively. CONCLUSIONS: Concerning the uncertainty in rectum definition, the collection of rectal dose-volume data in multicenter investigations seems to be feasible after a clear and previously accepted definition of rectum is assessed. However, even with a general agreement on rectum definition, contouring appears to be a quite significant source of uncertainty. A dummy run procedure is useful in identifying possible discrepancies among single observers and in assessing reliable confidence levels on dose-volume constraints because of contouring uncertainty, making the dummy run mandatory in multicenter trials evaluating 3D dose-volume data.