Variation of Clinical Target Volume Definition among Japanese Radiation Oncologists in External Beam Radiotherapy for Prostate Cancer

Background: We investigated the interobserver variation in the prostate target volume and the trend toward the use of diagnostic computed tomography (CT) or magnetic resonance (MR) images for treatment planning. Methods: Twenty-five radiation oncologists were asked to draw the external contour of the prostate on CT images (0.3 cm spacing) of a patient with localized prostate cancer. They also answered a questionnaire regarding the use of diagnostic CT or MR images for the contouring. Results: Of the 25 physicians, 28 % rarely or never referred to the diagnostic CT images. In contrast, the physicians tended to refer to the MR images more frequently. Approximately 50 % of the physicians believed in the usefulness of contrast-enhanced images for the deli-neation of the prostate. As for the variation of the prostate contouring, the median craniocau-dal prostate length was 36 mm (range, 21–54 mm), and the median prostate volume was 43.5 cm3 (range, 23.8–98.3 cm3). The interobserver variability was not significant in the dur