Is Belly Board Still Needed in IMRT for Rectal Cancer? The Change of Bladder Volume Affects the Determination

Abstract

Prone positioned with belly board is an efficiency method to reduce the irradiated small bowel volume (ISBV) in three-dimensional conformal radiation therapy (3DCRT) for rectal cancer patients. The downside of treating patients in prone position is that compared to supine, the setup is less reproducible between and during fractions. Intensity-modulated radiation therapy (IMRT) technique has previously been demonstrated to be effective in reducing small bowel dose in comparison to 3DCRT for rectal cancer. Whether the belly board still needed in IMRT for rectal cancer is not well answered. The aim of this study was to compare the ISBV between prone positioned with belly board and supine positioned with flat table in IMRT for rectal cancer patients. Twenty patients scheduled to receive postoperative radiation therapy for rectal cancer underwent two series of CT scans, prone positioned with a belly board and supine positioned with a flat board, both combined using a full bladder protocol. On each scan, a 7-field IMRT plan was calculated. Bladder and individual loops of small bowel were contoured on each slice of the planning CT scan. A comparison of volume for small bowel and bladder in different position were performed. Comparison of V15 (volume received dose more than 15 Gy) for small bowel was made between prone and supine position with different device. Compared with supine position, all bladder volumes were decreased in the prone position. The ISBV of eight patients were increased when use belly board. The increase of ISBV was related to the decrease of bladder volume. If the relative change of bladder volume was more than 16%, the ISBV might increase when use belly board. There were no significant differences between patients undergoing IMRT with belly board and those without in terms of V15 for small bowel and ISBV (p = 0.245 and p = 0.944, respectively). If we used 16% as cutoff value to select patients, V15 for small bowel and ISBV of different position could meet marginal statistical significant (p = 0.062 and p = 0.072, respectively). The change of bladder volume in two different positions affected the ISBV. If the change less than 16%, a belly board still attributed to a significant bowel dose reduction when using IMRT and a full bladder protocol.