A comparison of prone three-dimensional conformal radiotherapy with supine intensity-modulated radiotherapy for prostate cancer: which technique is more effective for rectal sparing?

The purpose of this study was to assess the potential dose reductions to the rectum with three-dimensional conformal radiotherapy in the prone position (prone 3D-CRT) compared with intensity-modulated radiotherapy in the supine position (supine IMRT) for prostate cancer. 17 prostate cancer patients underwent treatment planning CT scans in the supine and prone positions. Prone 3D-CRT and supine IMRT plans were constructed for each patient and compared in terms of the volume of rectum exposed to the V90 (volume of rectum receiving at least 90% of the prescription dose) as the high dose region. It was confirmed that supine IMRT was significantly superior to prone 3D-CRT (p = 0.023). Although, in some cases, the distance between the seminal vesicles and the rectum could change by more than 20 mm in the transition from supine to prone, the change in distance was approximately 5 mm in many other cases. While prone 3D-CRT resulted in significant improvements in some patients in terms of rectal sparing, the degree of the effect may be dependent on a patient's anatomy and physical condition in prone 3D-CRT compared with supine IMRT. If the cases in which prone 3D-CRT was more effective in rectal dose reduction could be extracted using some anatomical predictor before treatment planning, prone 3D-CRT may be appropriate in such a case. We consider that prone 3D-CRT still warrants further investigation because of its advantages in terms of simplicity, cost-effectiveness and labour saving; continued research to find an appropriate anatomical predictor is required.