Does prone positioning reduce small bowel dose in pelvic radiation with intensity-modulated radiotherapy for gynecologic cancer?

Purpose: Intensity-modulated radiotherapy (IMRT) has been shown to reduce the radiation dose to small bowel in pelvic RT in gynecology patients. Prone positioning has also been used to decrease small bowel dose by displacement of small bowel from the RT field in these patients. The purpose of this study was to determine whether the combination of both IMRT and prone positioning on a belly board can reduce small bowel dose further in gynecologic cancer patients undergoing pelvic RT. Methods and Materials: IMRT plans for pelvic RT were computed in 16 patients with gynecologic cancer who had undergone planning CT scans in both the supine and the prone positions on a belly board. For the gross tumor volume, the uterus, cervix, and tumor (or postoperative region) were traced. The clinical target volume was defined as the vessels and lymph nodes from the obturator level to the aortic bifurcation, presacral region, and upper 4 cm of the vagina, in addition to gross tumor volume. The planning target volume was defined as a 2-cm margin in addition to the gross tumor volume and upper 4 cm of the vagina, and 1.5 cm for lymph nodes and vessels. Normal tissue regions of interest included small bowel, large bowel, and bladder. IMRT plans using (1) the limited arc technique (180° arc length) and (2) the extended arc technique (340° arc length) were computed. Dose-volume histograms for normal tissue structures and target were compared between the supine and prone IMRT plans using the paired t test. Results: Prone positioning on a belly board decreased the small bowel dose in gynecologic pelvic IMRT, and the magnitude of improvement depended on the specific IMRT technique used. With the limited arc technique, prone positioning significantly decreased the irradiated small bowel volume at the 25-50-Gy dose levels compared with supine positioning. Small bowel volumes receiving ≥45 Gy decreased from 19% to 12.5% (p = 0.005) with prone positioning. With the extended arc technique, the decrease in irradiated small bowel volume was less marked, but remained detectable in the 35-45-Gy dose levels. Small bowel volumes receiving ≥45 Gy decreased from 13.6% to 10.1% (p = 0.03) with prone positioning. The effect of prone positioning on large bowel and bladder was variable. Large bowel volumes receiving ≥45 Gy increased with prone positioning from 16.5% to 20.6% (p = 0.02) in the limited arc technique and was unaffected in the extended arc technique. Conclusion: These preliminary data suggest that prone positioning on a belly board can reduce the small bowel dose further in gynecology patients treated with pelvic RT, and that the dose reduction depends on the IMRT technique used.