Proton Therapy


What is Proton Therapy?

When cancer affects sensitive areas of the body like the brain, spine and lungs, precise treatment is essential. Proton therapy is a highly advanced form of external radiation therapy that uses a of high-energy protons to target and destroys cancer cells with pinpoint accuracy. It focuses energy directly on the tumor while preserving the surrounding healthy tissue. This painless treatment option offers fewer side effects than traditional radiation therapy.

How Does Proton Therapy Work?

Traditional radiation therapy is delivered by photons (or x-rays). X-ray beams can continue beyond the tumor, releasing energy and possibly damaging nearby healthy tissues, known as the exit dose. Protons, in proton therapy, work differently. They can be charged to reach the exact depth where a tumor is located, then stop. Very little proton radiation continues past the tumor site, so there’s no exit dose.

During proton therapy, a powerful cyclotron device accelerates and charges the proton particles, so they can damage the DNA of cancer cells to keep them from growing and spreading. The protons can be focused directly on the cancer tumor, with very minimal side effects on the surrounding healthy tissue. With pencil beam scanning, proton beams can be focused to an even greater extent, resulting in a more precise radiation dose.

David C. Pratt Cancer Center
Proton Therapy St. Louis

Mercy’s MEVION S250iTM Proton Therapy System with the HYPERSCAN® Pencil Beam Scanning Accelerator is available to patients pursuing both research options and traditional care.

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Proton therapy improves the quality of life for people fighting cancer by:


  • Damaging the DNA of cancer cells, halting cancer in its tracks
  • Delivering targeted therapy that spares nearby healthy tissues
  • Allowing for higher doses of radiation to be delivered directly to tumors
  • Causing fewer side effects compared with traditional radiation therapy
  • Providing a painless and non-invasive treatment option

Proton therapy can treat both cancerous and noncancerous tumors in adults and children. It’s particularly effective at treating:

Childhood Cancers

Proton therapy treats a variety of tumors in children without damaging healthy organs and tissues. Children’s bodies are still developing, so they’re more sensitive to the effects of radiation therapy.

Brain Tumors

The brain contains delicate and critical structures that maintain key body functions. With proton therapy, treatment can be sculpted to the tumor, reducing the risk of damage to other brain tissues.

Head & Neck Tumors

Cancerous and noncancerous tumors can form in the head and neck, including in the nose, mouth, sinuses, throat, voice box, thyroid and other areas. Proton therapy effectively treats these areas without damaging their function.

Base-of-Skull Tumors

The skull base is the bottom part of the skull (or cranium) where the brain sits. The spinal cord and other vital nerves and vessels pass through it. Tumors can form inside the skull or extend into it. Proton therapy treats these tumors while preserving essential body functions like smell, taste, vision and swallowing.

Prostate Cancer

Proton therapy precisely targets prostate tumors with minimal damage to vital organs like the bladder and rectum. When less of the pelvis is exposed to radiation, men experience fewer side effects and enjoy a better quality of life.

Tumors Near the Spine

Cancerous and noncancerous tumors can develop within the spinal cord or on nearby bones and vertebrae. Proton therapy delivers precise radiation that conforms to spine tumors while reducing radiation to critical areas like the lungs.

Melanoma of the Eye

Proton therapy is the preferred treatment option for ocular melanoma (eye cancer). It delivers a powerful radiation dose to tumors while protecting the structures of the natural eye.

Lung Tumors

Treating lung cancer with traditional radiation therapy carries a greater risk of lung injury and radiation exposure to the heart. Proton therapy reduces radiation exposure risk to the lungs, esophagus and heart.

Pituitary Gland Tumors

The pituitary is a small gland located in the brain. Tumors in the gland can lead to vision problems, irregular hormone levels and other issues. Proton therapy effectively treats these tumors with reduced radiation side effects.

Arteriovenous Malformations (AVMs)

AVMs are abnormal tangles of blood vessels that can disrupt connections between the veins and arteries. Proton therapy helps treat these sensitive structures with reduced risk of bleeding, seizures or other issues.

Gastrointestinal Cancers

Treating gastrointestinal cancers (cancers of the colon, esophagus, pancreas and rectum) with radiation can affect nearby healthy tissues in the kidneys, liver, stomach and other areas. With proton therapy, radiation conforms to tumors while protecting surrounding organs and tissues.

Proton therapy is a preferred treatment option for children’s cancers because it protects their growing bodies from side effects. Excess radiation exposure can affect the function of kids’ hearts, lungs, kidneys, eyes, ears and reproductive organs.

Proton therapy also minimizes late side effects — these are long-term medical conditions that can develop after cancer treatment, such as learning disabilities, hearing loss and other issues.

Preparing for Proton Therapy

Proton therapy is a non-invasive, painless treatment given in an outpatient setting. The number of treatment sessions depends on cancer type and stage. Your Mercy cancer care team explains your treatment plan and the number of sessions required to effectively treat your cancer.

Positioning for Proton Therapy

Before treatment, you have a CT or MRI scan in the exact position you’ll be in during treatment. To reduce movement during the scan, you may be fitted with a special device. The type of device depends on where the tumor is in your body.


During a radiation planning scan, you lie on a table as the doctor plans where the radiation will be delivered to your body. This helps make sure your position is accurate during each proton therapy treatment. Your care team marks where the tumors are on your body using your scan. They also mark where normal tissues are to avoid exposing them to radiation.


Let your care team know if you feel anxious about staying in one position with the device. Your doctor may be able to give you medication to help you relax for the scans.

Proton Beam Treatment

Once you’re positioned, your care team leaves the room and goes to the control room. However, your team can still see and hear you through a video system.


During the actual proton therapy treatment, the protons travel through the machine, and magnets direct them to the tumor. It’s important to remain still during this process to avoid moving the tumor out of the focused proton beam.


In general, proton therapy lasts about 15 to 30 minutes, starting from the time you enter the treatment room. The time varies depending on the area of the body being treated and the number of treatments. It also depends on how easily the team can see the tumor with scans during the positioning process.

Recovery After Proton Treatment

The treatment is painless, but you may experience fatigue afterward. Some people also develop skin irritation. Ask your care team about symptoms you could experience after treatment.

Side effects depend on the part of the body being treated, tumor size and types of healthy tissues nearby. Skin irritation is a common side effect, including redness, swelling, dryness, blistering or peeling. If you’re also receiving chemotherapy, other side effects could apply. Talk with your care team to learn more.

Proton Therapy


Hear from Mercy's Dr. David Meiners sharing why proton therapy may be used for specific tumors.

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