Proton therapy is a form of radiation treatment which uses protons to deliver dose to cancer patients. Protons are tiny particles found in the nuclei of atoms, and they can be accelerated by specialized machines to high energies for radiation delivery.
Where do the protons come from?
The protons used in proton therapy come from hydrogen gas.
Hydrogen gas consists of hydrogen atoms, and each hydrogen atom is composed of one positively charged proton orbited by a single negatively charged electron. In hydrogen gas the atoms form hydrogen molecules, which are two hydrogen atoms bound together by shared electrons.
To obtain protons, a device known as an ion source strips the electrons from hydrogen molecules. The resulting free protons are then accelerated by a machine to high energies for use in treatment.
How are protons accelerated to treatment energies?
In order to treat a cancer deep within the body, protons must be given enough energy to penetrate to the target. This is done by accelerating the protons to high speeds.
After the electrons have been stripped from hydrogen atoms, the resulting free protons are injected into the accelerating stage of the proton therapy machine. There are various types of accelerating stages, but the most common in medicine are cyclotrons and synchrotrons.
In cyclotrons, protons are injected into the center of the cyclotron and forced by two strong magnets to travel in a spiral path. The magnets are divided into sectors and in gaps between sectors a voltage is established, creating an electric field. Each time the positively charged protons cross a voltage gap between magnet sectors they are attracted to the negative side of the voltage difference and gain energy from the electric field, causing the protons to increase their speed. As the protons accelerate, they travel in an arc of a larger circle, resulting in a widening spiral path. In synchrotrons, protons travel in a closed loop with a fixed size, and the magnetic field and voltage are continuously adjusted to accelerate the protons.
Protons emerging from a cyclotron or synchrotron can be controlled to have a chosen energy that enables them to be able to penetrate the patient’s body and reach the cancer to destroy it. Proton radiation doses, delivery times, and number of patient treatment sessions are similar to those used for x-ray and electron beam treatments. There are, however, differences in the energy of the radiation beam: For example, the energy required for a beam of protons to adequately treat deep-seated cancer is about 10 times higher than for a beam of electrons. This is because protons are much heavier than electrons and lose about 10 times more energy than electrons (per centimeter of penetration in tissue) before they stop.
Examples of proton therapy machines as seen inside the treatment room.
Image courtesy of Varian Medical Systems, Inc. All rights reserved.
Image courtesy of Medical Physics Graduate Programs, University of Pennsylvania.
How is proton therapy delivered?
The high-energy protons emerging from the accelerator travel down a pipe which is surrounded by magnets that keep the protons focused into a narrow beam and steer them to the treatment room. In the treatment room, a delivery system shapes and positions the proton beam according to the patient’s individualized treatment plan. This delivery system, which can be seen as the circular structure in the photos above, is called the treatment gantry and can rotate around the patient to aim the proton beam at the area being treated. There are different types of beam delivery systems in use but for each the goal is to control the energy of the beam incident on the patient so that the targeted tissue receives a high dose of radiation while surrounding normal tissues receive much less dose. This may require delivering several proton beams to the patient from different angles.
Why is proton therapy used?
The major advantage of proton beam therapy is that protons do not deliver radiation beyond a specific distance in the body. This is useful in protecting sensitive organs from radiation.
Protons have a well-defined depth of penetration that is dependent upon their energy. The maximum depth of penetration is called the range; as protons arrive at the range depth they slow down dramatically and give up most of their energy in a small distance. (This large spike in deposited energy is called the Bragg Peak in honor of physicist Sir William Bragg, a Nobel Prize winner in physics who first reported the phenomenon in 1903). Medical physicists are able to control the beam energy and range of proton treatment beams so that the highest doses are matched to the shape of the diseased site while healthy downstream tissues are spared from damage.
Comparison of radiation dose distributions from (left) a proton therapy plan and (right) an x-ray therapy plan. In the proton therapy plan all of the delivered dose remains close to the target (outlined in red), whereas in the x-ray plan there is a large region of low dose at distances from the target.
