Proton Technology
Each proton begins its journey at an injector located within an electric field. In the field, hydrogen atoms are separated into negatively charged electrons and positively charged protons. The protons are then sent through a vacuum tube within a pre-accelerator. This process boosts their energy to two million electron volts.
The protons continue in the vacuum tube and begin their high-speed journey in the synchrotron. They travel around the synchrotron about 10 million times per second. Each time they circulate, a radiofrequency cavity within the ring delivers a boost of energy. This increases the protons' energy to between 70 and 250 million electron volts. The voltage achieved is enough to place them at any depth within the patient's body.
After leaving the synchrotron, the protons move through a beam transport system, continuing in the vacuum tube through a series of steering and focusing magnets that guide them to the four treatment rooms. (A fifth room, used for beam calibration and basic research, contains three additional beam lines.) One treatment room has a stationary beam with two branches. One branch is for irradiating eye tumors and the other branch is for central nervous system tumors and tumors of the head and neck. The other three treatment rooms have gantries. Gantries are wheels 35 feet in diameter and weighing approximately 90 tons, which revolve around the patient to direct the beam precisely to where it is needed. From the patient's perspective, however, all that is visible is a revolving, cone-shaped aiming device.
Each treatment room has a guidance system to direct the beam that treats the patient. The guidance system monitors the beam until it enters the patient and positions the Bragg peak to conform to the size and shape of the tumor and the immediate volume at risk for tumor spread, according to a plan designed by the physician. Field sizes to accommodate regional spread of tumors can be as large as 40 by 40 centimeters.
The beam delivery system, or nozzle, is the last device the protons travel through before entering the patient's body. The nozzle shapes and spreads out the proton beam in three dimensions. When calculating the number of protons to be delivered to the designated volume and the velocity and shape of the proton beam, radiation oncologists take into account the location and shape of the target and the tissue density and depth through which the protons must travel to reach their target. After leaving the nozzle, the protons enter the patient's body.
The entire proton facility is controlled by computers that are equipped with safety measures to ensure that patients receive proton radiation treatments as prescribed. Three host computers control the accelerator and beam transport system. A second set of computers, located in each treatment room, controls the beam guidance system and all other treatment room systems. These computers verify patient identification, set operational parameters for each patient's treatment plan, and direct the host computers to deliver a specified beam.
All this is done at Loma Linda to make in as rigorous and safe a manner as possible. In this way, the superior benefits of proton therapy can be offered to more patients with different types of cancers.