PURPOSE: We conducted a comparative study of clinical target volume (CTV) definition of pelvic lymph nodes by multiple genitourinary (GU) radiation oncologists looking at the levels of discrepancies amongst this group. METHODS AND MATERIALS: Pelvic computed tomography (CT) scans from 2 men were distributed to 14 Radiation Therapy Oncology Group GU radiation oncologists with instructions to define CTVs for the iliac and presacral lymph nodes. The CT data with contours were then returned for analysis. In addition, a questionnaire was completed that described the physicians' method for target volume definition. RESULTS: Significant variation in the definition of the iliac and presacral CTVs was seen among the physicians. The minimum, maximum, mean (SD) iliac volumes (mL) were 81.8, 876.6, 337.6 +/- 203 for case 1 and 60.3, 627.7, 251.8 +/- 159.3 for case 2. The volume of 100% agreement was 30.6 and 17.4 for case 1 and 2 and the volume of the union of all contours was 1,012.0 and 807.4 for case 1 and 2, respectively. The overall agreement was judged to be moderate in both cases (kappa = 0.53 (p < 0.0001) and kappa = 0.48 (p < 0.0001). There was no volume of 100% agreement for either of the two presacral volumes. These variations were confirmed in the responses to the associated questionnaire. CONCLUSIONS: Significant disagreement exists in the definition of the CTV for pelvic nodal radiation therapy among GU radiation oncology specialists. A consensus needs to be developed so as to accurately assess the merit and safety of such treatment.