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The Sciences

Interactive: How Proton Therapy Works

A step-by-step illustration of the burgeoning technology for cancer treatment.


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PROTON THERAPY starts with accelerators housed in concrete bunkers several stories tall and large enough to hold a cyclotron. Giant magnets speed subatomic particles toward the speed of light.

Atom Smashers for Tumors

1. The proton beam begins inside a cyclotron, where enormously powerful magnets strip protons from the hydrogen atoms in water molecules.

2. Radiofrequency power accelerates protons in a circular path inside the cyclotron, a machine just slightly bigger than an SUV. The protons accelerate toward the speed of light, at 100,000 miles per second.

3. When protons reach their top speed, they’re directed out of the cyclotron by magnets into the beamline – an airless tube the length of a football field.

4. A series of magnets shape, focus and direct the proton beam to the appropriate treatment room.

5. A 90-ton gantry, in which the patient lies, helps aim the proton beam at specific targets on the body. The beam flows through a nozzle, with its shape and energy matching the 3-D profile of the tumor, and into the patient’s body.


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