PURPOSE: To evaluate the efficacy of a belly-board device (BBD) in reducing the volume of small bowel during four-field pelvic irradiation. MATERIALS AND METHODS: Twenty-two cancer patients (14 uterine cervical cancer, 6 rectal cancer, and 2 endometrial cancer) scheduled to receive pelvic irradiation were selected for this study. Two sets of CT images were taken with and without the belly-board device using the Siemens 16 channel CT scanner. All patients were set in the prone position. The CT images were transferred to a treatment planning system for dose calculation and volume measurements. The external surfaces of small bowel and the bladder were contoured on all CT scans and the 4-pelvic fields were added. The dose-volume-histogram of the bladder and small bowel, with and without the BBD, were plotted and analyzed. RESULTS: In all patients, the total small bowel volume included in the irradiated fields was reduced when the BBD was used. The mean volume reduction was 35% (range, 1~79%) and was statistically significant (p<0.001). The reduction in small bowel volume receiving 10~100% of the prescribed dose was statistically significant when the BBD was used in all cases. Almost no change in the total bladder volume involved was observed in the field (<8 cc, p=0.762). However, the bladder volume receiving 90% of the prescribed dose was 100% in 15/22 patients (68%) and 90~99% in 7/22 patients (32%) with the BBD. In comparison, the bladder volume receiving 90% of the prescribed dose was 100% in 10/22 patients (45%), 90~99% in 7/22 patients (32%), and 80~89% in 5/22 patients (23%) without the BBD. When the BBD was used, an increase in the bladder volume receiving a high dose range was observed. CONCLUSION: This study shows that the use of a BBD for the treatment of cancer in the pelvic area significantly improves small bowel sparing. However, since the BBD pushed the bladder into the treatment field, the bladder volume receiving the high dose could increase. Therefore it is recommended to be considerate in using the BBD when bladder damage is of concern.