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What is Proton Therapy

Proton therapy is an extremely precise form of radiation treatment. Most radiation treatments use X-rays, but proton therapy uses protons, which are tiny particles from the center of an atom. Compared to a beam of X-rays, a proton beam can better concentrate the radiation in the target, conforming more closely to the shape and depth of a tumor and avoiding normal, healthy tissues. Another unique feature is that after treating the target, protons come to a complete stop, meaning they deliver no radiation dose to areas beyond the target, resulting in less radiation to healthy tissues around the tumor.

Because proton beams can reduce or eliminate radiation to sensitive normal healthy tissues and organs, proton therapy may reduce some of the side effects that can occur as a result of radiation treatments, and may reduce some potential long-term side effects and risks that can occur months or years after radiation treatment.

Proton Therapy vs. X-Ray Radiation

What is Pencil Beam Proton Therapy?

Pencil-Beam Proton Therapy is a type of radiation therapy that:

  • Uses protons, positively charged subatomic particles with mass
  • Deposits less radiation on the path to the target with most of the dose in the target
  • Stops releasing radiation energy at the target 
  • May result in less damage to healthy tissues & organs

What is X-Ray Radiation?

X-Ray Radiation is a type of radiation treatment that:

  • Uses photons (x-rays) with no mass & no charge
  • Can deposit more radiation in the entry path compared to the dose which reaches the target
  • Continues to release radiation energy beyond the target, along the entire path through the body
  • May result in an increase in side effects and risks to healthy tissues & organs

Why Choose Proton Therapy

There are several benefits of proton therapy as compared to other forms of cancer treatment. Benefits include reduced radiation exposure, reduced side effects, reduced long-term effects, no pain, ability to treat complex and recurrent tumors, and ability to treat children.

Reduced Radiation Exposure

Proton beam therapy with pencil beam scanning can reduce or avoid unnecessary radiation to normal tissues and organs surrounding the target.

Reduced Side Effects

By reducing or avoiding radiation to normal areas, proton therapy may reduce some side effects during and after the treatment course, like difficulty swallowing for head and neck patients, urinary or bowel problems for prostate cancer patients and inflammation or damage to the heart or lung for treatment of the chest or breast.

Reduced Long-Term Effects

Reduced exposure to radiation reduces the risks of possible longer-term effects from treatment, such as dry mouth, damage to the the heart or lung, or secondary cancers, with a goal of improved quality of life.


Like other forms of radiation, proton therapy treatment delivery is invisible and painless. Fewer side effects may allow more patients to maintain their work and family commitments through treatment and to recover sooner.

Treatment of Complex Tumors

Proton therapy is often used to treat complex or irregularly shaped tumors that are in close proximity to vital organs, critical structures, or important healthy tissue.

Treatment of Recurrent Tumors

In some cases, proton therapy may be used to treat tumors that have recurred after prior radiation treatment, a situation in which other types of radiation may not be a good option.

Treatment of Children

Proton therapy reduces the risks of radiation in children, whose growing bodies are most at risk of radiation side effects.

The Technology of Proton Therapy

Proton therapy is administered through a complex and intricate array of highly advanced technology.


Proton therapy starts in the 90-ton superconducting cyclotron, which accelerates protons to about 60% of the speed of light, which is more than 110,000 miles per second. Going that fast, one could travel around the earth more than 4 times in less than a second! At this speed, the protons have the energy to reach tumors even if they are deep in the body.

Cyclotron-Technology Behind the Scenes

Energy Selection Source

The energy of the proton beam is adjusted to match the needs of each patient’s treatment based on the depth of the target.


The proton beam travels through a long vacuum tube, called the beamline, to reach each treatment room. Special magnets are used to steer the proton beam and keep it focused as it travels to each treatment room.



Although not visible from inside the treatment room, each proton gantry is nearly three stories tall and weighs about 270 tons. This large structure can rotate 360 degrees around the patient so that the proton beam can be directed from any direction required.

The proton gantry is equipped with low-dose x-ray panels that can also acquire cone-beam CT scans to ensure you are in the proper position and the proton beam is aimed correctly prior to treatment delivery.

Wide view of cyclotron

Cone Beam CT Imaging

Two x-ray tubes and imaging panels are mounted on the gantry and can be used to obtain cone beam CT scans just prior to treatment, which allows the team to verify the patient is correctly aligned prior to proton delivery, and to evaluate changes during treatment.

Fixed-Beam Room

The fixed-beam room is specially designed primarily for the treatment of patients with prostate cancer. The proton beam comes out of the wall at a fixed position and cannot rotate. A robotic patient positioner allows the patient to be rotated in front of the beam to treat from the desired treatment angles.

The fixed beam room is equipped with low-dose X-ray panels that are used to ensure you are in the proper position and the proton beam is correctly aimed prior to treatment delivery.

Pencil Beam Scanning

All of our treatment rooms use the most advanced pencil beam scanning technology (also known as Intensity Modulated Proton Therapy or IMPT). A narrow stream of protons is steered by a magnet to “paint” the radiation to match the size, shape, and depth of the target. This allows for maximum flexibility in treatment planning and sparing of healthy tissues and surrounding organs.

Is Proton Therapy Right For Me?

The Emory Proton Therapy Center provides patients with the latest proton treatment technology combined with the expertise of renowned specialists at Winship Cancer Institute of Emory University. Patients benefit from extensive and multidisciplinary evaluation and treatment, access to clinical trials, and other advanced therapies and approaches.

The best way to determine if proton therapy is a good treatment option for you is to meet one of our radiation oncologists in consultation or be seen in one of our multi-disciplinary clinics.  During a consultation visit, your doctor will work with you to develop an overall plan of care, discuss all radiation treatment options, including proton therapy, discuss the details of treatment, including the potential risks and side effects, and answer questions.

Proton therapy is merely one of Emory’s cancer treatment tools at Winship. Our multidisciplinary team will look at your case, compare the benefits and risks of different treatment options, and then recommend what may work best for you.

Male doctor with female patient

Second Opinion

We understand that choosing a healthcare provider can be a challenging decision. If you or your loved one are undecided about the plan of care, we offer second opinion consultations both at Emory Proton Therapy Center and in our multi-disciplinary clinics within Winship.

In order to receive a second opinion, please call our main line at 833-377-6866 so that we can register you and collect necessary medical records. These medical records will be reviewed by one of our Radiation Oncologists who have expertise with that specific disease site. Telehealth consultations are available for appropriate patients in Georgia and certain other states.  If you decide to pursue proton therapy, a comprehensive in-person consultation will be scheduled. 

The History of Proton Therapy

In the early 1900s, Nobel-prizewinning physicist Ernest Rutherford discovered the nucleus or center of an atom and proposed the name “proton” for the positively-charged particles in the nucleus.  Another Nobel-prize winning physicist William Bragg described the unique way that particles, like protons, deposit their energy. Unlike photons (X-rays), which have decreasing energy deposition with increasing depth of penetration, particles like protons have a sharp increase in energy deposition near the end of their range followed by a sharp drop off. This phenomenon is called the Bragg peak in his honor, and is the reason why proton therapy often results in less radiation to normal tissues outside of targets compared to photon therapy.

In 1930, Ernest Lawrence, yet another Nobel-prize winner, invented the cyclotron, which could generate a high-energy stream of protons and greatly advanced research in particle physics.  The cyclotron continues to be widely used in medicine, for example to create radionuclides for imaging and for proton therapy. The physicist Robert Wilson, who studied with Dr. Lawrence, proposed the use of proton therapy for treatment of cancer in a 1946 paper, describing the advantages compared to photons and outlining many of the early techniques used for proton treatment.  The first proton therapy patient treatments were begun in 1954 in nuclear physics research facilities.

Ernest Lawrence adjusting the ion source of a cyclotron

For many decades, proton therapy remained limited to just a few centers and was often used to treat rare and challenging tumors. The U.S. Food and Drug Administration approved proton therapy in 1988. Improvements in imaging, engineering and computer technology enabled the development of pencil-beam scanning proton therapy, which greatly improved upon early forms of proton therapy and increased the interest in proton therapy to treat more diseases.  These improvements led to more widespread investment in proton therapy technology in the US and across the world to serve more patients. In 2005, there were just four proton centers operating in the United States, which grew to 10 by 2010, and as of 2021 there are 37. According to statistics compiled by the Particle Therapy Co-Operative Group, as of the end of 2019, proton therapy had been used in the treatment of over 222,000 people worldwide. The Emory Proton Therapy Center treated its first patients on December 4, 2018.