Development of an irradiation method of intraoperative radiation therapy for curatively resected rectal cancer

Background. Intraoperative radiation therapy (IORT) has been performed to prevent local recurrence of rectal cancer only when positive margins are suspected. To further reduce local recurrence, we attempted to develop a new IORT irradiation method in which electron beam irradiation is administered as uniformly as possible to the intrapelvic dissection surfaces. Methods. Low anterior resection and abdominoperineal resection were performed in one male and one female cadaver. Electron beam irradiation was administered by four different methods, and absorbed doses were measured at 15 sites within the pelvis. We also attempted to measure absorbed doses at nine sites within the pelvis in 14 patients treated with IORT. Results. The cadaver study revealed low absorbed doses in the lateral walls of the pelvis when a single irradiation was delivered from the anterior. When the lateral walls of the pelvis were irradiated twice, once each time on the right and left, the absorbed doses were low in the central pelvis and presacrum. Relatively high absorbed doses were achieved in all of these areas by a technique that combined these two methods. Adequate absorbed doses were not achieved by a single irradiation administered from the perineum. Conclusion. This study suggests that electron beam irradiation administered three times to the dissected surfaces in the pelvis after resection of rectal cancer (i.e., to the central pelvis and presacrum from the anterior, and to the left and right lateral walls of the pelvis) is the most suitable method for achieving adequate absorbed doses.