When the use of protons as a cancer-killing agent was first theorized by physicist Robert R. Wilson in the mid-1940s, the technology had not yet been developed that could harness and direct these particles effectively. Nor did medicine have the capabilities to accurately locate tumors within the body.
After 30 years of technological advances, proton therapy’s potential to treat patients with less damage to healthy tissue became a reality in the late 1970s. Today, highly sophisticated accelerators enable protons to achieve the energies required to penetrate deep enough in the body to reach all types of tumors. And enhanced delivery systems enable treatment providers to pinpoint treatment to cancer tissues with the highest degree of precision.
Both conventional radiation and proton radiation kill cancer cells by damaging their DNA—preventing them from growing and dividing. The differences between the two treatments result from their physical properties.
Conventional X-rays are electromagnetic waves that have no mass or charge and penetrate completely through tissue. X-rays release their maximum dose of radiation quickly after penetrating the skin. Thus, they damage healthy tissue and organs on their way to the tumor and again as they pass through the body beyond the tumor.
Protons, on the other hand, are large, positively charged particles that can be conformed to release their cancer-killing energy with great precision. This enables radiologists to deposit most of the radiation exactly at the tumor site, regardless of whether the tumor is near the surface of the skin or deep inside the body—significantly limiting radiation exposure to surrounding normal tissues.
Protons enter the patient at a low dose, and then deliver a large burst of energy at the tumor. Immediately after this burst, called the Bragg peak, the radiation dose falls to nearly zero. To treat the entire tumor, multiple bursts are directed at different distances to provide a complete distribution of radiation to the tumor when added together. In this way, protons completely irradiate the tumor while minimizing damage to healthy tissue. And, because more energy can be deposited directly in the tumor, a higher dose typically can be delivered more safely in treating cancers that may benefit from an accelerated-dose approach.
The way in which proton therapy can be precisely delivered is illustrated below: