What Is the Bragg Peak?

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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 24 October 2019
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The Bragg peak is a point of increased concentration as radiation moves through a patient’s tissue. Studies on ionizing radiation show that it slowly loses energy as it travels, and the energy loss can be graphed by depth. At first the rate may remain consistent, but it drops off sharply right before the particles come to rest. This finding has important implications for radiotherapy, where the goal is to target tumor cells while protecting healthy tissue to limit side effects for the patient.

Researcher William Henry Bragg discovered the Bragg peak in 1903 while conducting studies on radiation. His work was integrated into protocols developed to guide the use of radiation in cancer therapy. Knowledge about the Bragg peak allows technicians to very precisely calibrate and target a radiation beam to hit the tumor at the right spot. They use imaging studies and other diagnostic tools to find out the depth and dimensions of the tumor so the beam can be properly aimed.

Radiotherapy can also use specific tools to change the shape and structure of the Bragg peak. The beam may be passed through another medium on its way to the patient, for example, to make the peak longer. This can help with full penetration of a large tumor, allowing the beam to cover the whole growth instead of just part of it. Computer programs help technicians calibrate the equipment and program the beam to use ionizing radiation as safely and effectively as possible.


When the Bragg peak is calibrated correctly, the beam deposits a high dose of radiation in exactly the right spot. As it travels through the tissue in front of the tumor, some deposition of radiation will occur, but it should remain much lower than the peak. The radiation stops shortly past the boundaries of the abnormal growth, ensuring that it doesn’t travel into surrounding healthy tissue. It can be difficult to calibrate in close environments with low margins for error, like the gut, where radiation therapy can come with a risk of perforation and inflammation.

Patients preparing for cancer treatment may receive a number of tests and evaluations before treatment starts. These can help shape a treatment tailored to the specific cancer to increase the chance of success. It is important to follow directions during testing to make sure technicians get the right measurements, as otherwise the equipment used to deliver radiation therapy might be improperly programmed.


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