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Why does proton beam therapy have little or no radiation dose beyond a tumor?

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linear accelerator; external beam radiation therapy — radiation therapy External beam radiation therapy (also known as external beam teletherapy, or long-distance therapy) being delivered using a machine known as a linear accelerator.

Proton beam therapy minimizes radiation exposure to healthy tissue beyond a tumor due to a physical phenomenon known as the Bragg peak.1

Unlike photon radiation, which continues to pass through and deposit energy in tissues beyond a target, charged proton beams deposit relatively low energy as they enter the body. As the particles penetrate deeper, they deposit increasing amounts of energy, reaching a sharp maximum at the end of their range—the Bragg peak—where the remaining energy is released over a very short distance.1

Immediately following this peak, the radiation dose drops rapidly to zero. This characteristic allows clinicians to deliver a high, uniform dose directly to the tumor site while sparing critical structures located behind it, making it a highly precise option for treating sensitive areas like the spinal cord or pediatric cancer cases.1