damage to normal tissues. IORT treatments with low energy or "orthovoltage" X-rays gained advocates throughout the 1930s and 1940s, but the results were inconsistent. The X-rays penetrated beyond the tumor bed to the normal tissues beneath, had poor dose distributions, and took a relatively long time to administer. The technique was largely abandoned in the late 1950s with the advent of megavoltage radiation equipment, which enabled the delivery of more penetrating external radiation. In 1965, the modern era of IOERT began in Japan at Kyoto University where patients were treated with electrons generated by a betatron Compared with other forms of IORT such as orthovoltage X-ray beams, electron beams improved IOERT dose distributions, limited penetration beyond the tumor, and delivered the required dose much more rapidly. Normal tissue beneath the tumor bed could be protected and shielded, if required, and the treatment took only a few minutes to deliver. These advantages made electrons the preferred radiation for IOERT. The technique gained favor in Japan. Other Japanese hospitals initiated IOERT using electron beams, principally generated from linear particle accelerators. At most institutions, patients were operated on in the operating room (OR) and were transported to the radiation facility for treatment. With the Japanese IOERT technique, relatively large single doses of radiation were administered during surgery, and most patients received no follow-up external radiation treatment. Even though this reduced the overall dose that could potentially be delivered to the tumor site, the early Japanese results were impressive, particularly for gastric cancer. The Japanese experience was encouraging enough for several U.S. centers to institute IOERT programs. The first one began at Howard University in 1976 and followed the Japanese protocol of a large, single dose. Howard built a standard radiation therapy facility with one room that could be used as an OR as well as for conventional treatment. Because the radiation equipment was also used for conventional therapy, the competition for the machine limited the number of patients that could be scheduled for IOERT. In 1978, Massachusetts General Hospital (MGH) started an IORT program. The MGH doctors scheduled one of their conventional therapy rooms for IOERT one afternoon a week, performed surgery in the OR, and transported the patient to the radiation therapy room during surgery. This used the radiation equipment more efficiently and required no additional capital outlay. However, about 30-50% of the patients planned for IOERT were found to be unsuitable