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 delineation of the prostate. As for the variation of the prostate contouring, the median craniocaudal 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 duration as a radiation oncologist, the board certification status as radiation oncologists, and the number of treatment plans developed for prostate cancer during the last 1 year. Conclusion: A wide variety of the definitions of the prostate was found among Japanese radiation oncologists.