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Photons and protons are small particles that both deliver radiation to destroy cancer cells. But they have different ways of hitting their targets. Doctors may recommend newer proton radiation therapy or traditional photon radiation therapy depending on which one is the most effective fit for your specific cancer.

¡°There are certain cases where protons are better and others where photons are more appropriate. A lot of research is going on to better understand who should be treated with one method or the other,¡± said Theodore Hong, MD, a radiation oncologist at Dana-Farber Cancer Institute and Beth Israel Deaconess Medical Center. Dr. Hong is also an expert.

What¡¯s the difference between proton and photon radiation therapy?

Radiation therapy is a core cancer treatment. It helps cure about 2 out of every 5 cancers worldwide. Both photon and proton therapies use high-energy beams to target a tumor. These beams damage the DNA inside the cancer cells. Because the cells cannot repair the damage easily, they die off.

¡°A key difference between the treatments is what we call the exit dose. That¡¯s the radiation to everything behind the tumor,¡± said Dr. Hong. Because of how photons work, photon therapy delivers an exit dose. But proton therapy does not.

Photon therapy, also called photon beam therapy, is currently the standard. Photons are high-energy x-rays that work like light. They pass completely through tissues, like x-rays that take pictures of bones. Photon beams can damage healthy cells both in front of and behind the tumor.

Proton therapy, also called proton beam or particle beam therapy, works differently. Protons are particles with mass and a positive electrical charge. As protons travel through tissue, they bump into things and slow down. Doctors can program proton beams to stop inside a tumor, which protects the healthy tissue on the other side.

Pros and cons of photon therapy

Photon therapy offers a number of benefits.

• Proven success: Doctors have used photons for decades and have evidence that they work.
• High availability: Almost every cancer center has photon machines.
• Lower cost: Photon treatment costs less and is usually covered by insurance.

The main risk of photon therapy is unwanted radiation exposure to healthy tissues. However, ¡°advances in photon therapy have really closed the gap between protons and x-rays,¡± said Dr. Hong. Even with improved accuracy, though, x-rays still travel through the target and can affect healthy cells on the other side.

Pros and cons of proton therapy

The main benefit of proton therapy is minimizing the effects of radiation on healthy tissue. This matters most for children and young adults who have decades of life ahead. Because proton therapy is more targeted, doctors may be able to use higher doses of radiation. However, studies on increased doses have shown mixed results. Whether more radiation helps will likely depend on the specific type of tumor.

The downsides of proton therapy include:

• Limited access: Very few hospitals have proton machines. Many people must travel long distances to get this treatment.
• High cost and insurance denials: Proton treatment is expensive. Because traditional photon treatment costs less and works well for most common cancers, health insurance companies often deny coverage for proton therapy at first. Going through an appeal process is a serious hurdle that can delay care.

When is proton therapy used?

Doctors recommend proton therapy instead of photon therapy when it¡¯s critical to protect nearby healthy tissue. For example:

• Childhood cancers: A child¡¯s growing body is very sensitive to radiation. Protons can reduce the risk of long-term health problems later in life.
• Bone tumors at the base of the skull (chordoma and chondrosarcoma): Protons can deliver the high doses of radiation needed to treat these cancers while limiting damage to nearby tissues that control vital body functions.
• Eye cancer: Precise proton beams can target a tumor while saving the eye and protecting vision.
• Repeat treatments: If an area needs radiation again, proton therapy can help prevent the buildup of DNA damage in nearby tissue.

What does the research say?

For many common cancers, such as prostate and breast cancers, standard photon therapy seems to be just as effective as proton therapy. For example, a looked at more than 2,500 people with prostate cancer. Early results presented at ASCO¡¯s 2026 Annual Meeting showed that both treatments successfully controlled the cancer, with similar short-term side effects. Further studies are being done to help learn about long-term side effects and outcomes.

However, some recent trials suggest that proton therapy may be more effective, particularly for tumors near critical organs. This includes certain cancers in the brain, spine, head and neck region, or pelvis. Because proton beams are more targeted, the surrounding healthy tissues are less affected. As a result, people may have fewer severe side effects or live longer.

The full picture of how these therapies compare over the long term is still unclear. Late effects, such as second cancers, heart damage, thinking (cognitive) impact, and hormone issues, can take years to show up. Ongoing research will help explore these differences.

¡°Understanding who protons are better for is turning out to be a much more sophisticated issue than we previously realized, and the research is evolving,¡± said Dr. Hong.

Both types of treatment are excellent tools. The right choice depends on your specific tumor. Dr. Hong recommends seeking an opinion from a doctor who has deep experience with proton therapy for your specific cancer. ¡°Sometimes a doctor will know pretty quickly that protons aren¡¯t needed, and that¡¯s a good answer to get as well,¡± said Dr. Hong.

Learn more from the PÕ¾ÊÓƵ:

• Understanding Your Options for Cancer Treatment
• What Goes into Planning Your Radiation Therapy
• Understanding Information About New Cancer Treatments

Dr. Hong is a member of ASCO¡¯s .