So far, I’ve spoken about subatomic particles, beams of energy, and eradicated cancers, but how does proton therapy actually work? What technology is used to treat tumors? Is it safe? Is it effective?
The best way to understand how proton therapy works is to look at the physics and engineering behind the proton accelerator, called a synchrotron, and the beam delivery system that work in conjunction to eliminate cancerous cells. After all, this treatment was the brainchild of a physicist.
Each proton begins its journey at an injector located within an electric field, known as the ion source. Within fractions of a second, 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. In a matter of microseconds, the protons’ energy reaches seven million electron volts.
Proton beams stay in the vacuum tube as they enter the synchrotron, where acceleration increases their energy to a total of 70 million to 250 million electron volts, enough to place them at any depth within the body. Once they leave the synchrotron, the protons move through a beam-transport system comprised of a series of magnets that shape, focus, and direct the proton beam to the appropriate treatment room. Having specific treatments rooms is the standard operating practice for proton therapy treatment centers. At the Loma Linda Center, for example, there is one room that contains a stationary beam with two branches. One branch is for irradiating eye tumors and the other branch for tumors in the central nervous system, head, and neck. The other three treatment rooms in the facility have gantries, wheels that are 35 feet in diameter and weigh approximately 90 tons, which revolve around the patient to direct the proton beam precisely where it is needed. The machine at the MD Anderson Cancer Center weighs 196 tons and features a gantry that can revolve 360 degrees, allowing the beam to be delivered at any angle. Proton beam technology has come a long way since the 1950s!
While a patient lies comfortably on a bed, protons come through a nozzle and a shaping device, called an aperture, shapes the bean while another device, called a compensator, spreads the protons into three dimensions and conforms it to the depth of the tumor. Proton therapy facilities use computers to control the dose and accuracy of the proton beam. This helps insure that the patient receives the prescribed proton treatment.
At maximum energy, a proton beam travels 125,000 miles per second, equivalent to two-thirds the speed of light. From the hydrogen canister to the patient, a proton typically travels 313,000 miles. During the whole procedure, the patient feels no pain or discomfort.
Proton therapy can take anywhere from one day to seven weeks, depending on the tumor site. The length of treatment time will also decrease over time as heavier doses begin to increase.
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