A performance study of the Loma Linda proton medical accelerator

More than three years have passed since Loma Linda treated the first cancer patient with the world's first proton accelerator dedicated to radiation therapy. Since that time, over 1000 patients have completed treatments and the facility currently treats more than 45 patients per day. With a typical intensity of 3 x 10(10) protons per pulse and 27 pulses per minute, dose rates of 90-100 cGy/min are easily achieved on a 20-cm diameter field. In most cases, patient treatment times are 2 min, much less than the patient alignment time required before each treatment. Nevertheless, there is considerable medical interest in increasing field sizes up to 40-cm diameter while keeping dose rates high and treatment times low. In this article, beam measurements relevant to intensity studies are presented and possible accelerator modifications for upgrades are proposed. It is shown that nearly all intensity losses can be ascribed to the large momentum spread of the injected beam and occur at or near the injection energy of 2 MeV. The agreement between calculations and measurements appears quite good. In addition, optimum beam characteristics for a new injector are discussed based upon the momentum acceptance and space charge limits of the Loma Linda synchrotron.