Intraoperative radiotherapy, which delivers a large single dose of electrons to a tumor intraoperatively, is applied for locally advanced and/or difficult to cure malignancies which are rather difficult to treat by conventional external beams. Although this method was initially developed in Japan, increasing numbers of institutes have recently been working with IORT on a world-wide basis because of the increased availability of medical accelerators and the better results obtained. The malignancies for which IORT is indicated are cancers of the stomach, liver, biliary tract, pancreas, colo-rectum, urinary bladder, prostate and uterus, which are surrounded by normal tissues or organs with low tolerance doses, and also recurrent or difficult to cure malignancies of the brain, lung, mediastinum, bone and soft tissues are becoming candidates for IORT treatment.