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What Is Proton Therapy?

Proton therapy is an effective form of radiation therapy for many types of tumors. It destroys cancer cells by preventing them from dividing and growing, just like standard X-ray (photon) radiation.

Listen to Dr. Eugen Hug talk about Proton Therapy

The difference between proton therapy and standard X-ray radiation is that protons deposit much of their radiation directly in the tumor and then stop. This allows patients to receive higher doses, which can be more effective, while reducing damage to healthy tissues that surround the tumor.1,2

The most significant benefit of proton therapy over standard X-ray radiation is fewer short- and long-term side effects.3-8

When compared to conventional X-rays and intensity-modulated radiation therapy (IMRT), proton therapy significantly decreases the likelihood of secondary malignancies. Researchers analyzed the records of 1591 patients treated with X-ray radiation and compared them with 503 patients treated with proton therapy. Of the patients treated with X-ray radiation, 12.8% of them developed secondary malignancies, while only 6.4% of the proton therapy patients developed secondary malignancies.5

How does it work?

When used to treat a tumor, X-rays and protons deliver radiation in different ways, because the physical properties of X-rays are different from the properties of protons. X-rays are electromagnetic waves that have no mass or charge and penetrate completely through tissue. Protons are large, positively charged particles that penetrate matter to a finite depth.1

Protons can be conformed to release much of their energy at precise depths so they can target tumors inside the body, depositing much of their radiation exactly at the tumor site.1

X-rays release their maximum dose of radiation soon after penetrating the skin, potentially damaging healthy tissue and organs on their way to the tumor and again as they pass through the body beyond the tumor.1


Click image above to enlarge.

Imagine that the Y axis (relative radiation dose) in the figure above is the surface of a patient’s skin and the beige rectangle is a tumor. The goal of treatment is to deliver the proper dose of radiation to the tumor while limiting the dose received by the surrounding healthy tissue.

The sloping red line shows how X-rays/IMRT deliver a dose. To deposit the proper amount of energy into the tumor, X-rays/IMRT must irradiate much of the healthy tissue in front of it, and they continue to penetrate through the tumor and irradiate much of the healthy tissue behind it.

Protons deliver their dose in a very different way. As the turquoise line shows, they enter the patient at a low dose, then, at a precise depth, they deliver a large burst of energy. Immediately after this burst, they stop completely. To treat the entire tumor, additional protons are sent in at lower doses. In this way, protons completely irradiate the tumor while limiting the dose to the nearby healthy tissue.

The areas shaded in red show the additional dose delivered to healthy tissue by X-ray/IMRT compared with protons. Proton treatment delivers a dose in a more accurate, more efficient way and spares more of the surrounding healthy tissue.

The most significant benefits of proton therapy1-8

When compared to standard X-ray radiation:

  • Reduced risk of damage to healthy tissue
  • Fewer short- and long-term side effects
  • Improved quality of life
  • Lower incidence of secondary tumors

Is proton therapy new?

Proton therapy is not new. The first patient received treatment with protons more than 50 years ago, and the U.S. Food and Drug Administration approved proton therapy as a radiation treatment option in 1988. To date, more than 90,000 people worldwide have received proton therapy at centers in Europe, Asia, and the United States. It is covered by many private insurance companies, Medicare, and many state Medicaid programs.9

Why you may not have heard of proton therapy

Many doctors are not familiar with proton therapy because of the limited number of treatment facilities in the United States. ProCure is working to increase this number and is dedicated to making proton therapy accessible to everyone who needs it.

Today there are nine proton therapy centers in operation with at least five more in development.

More FAQs »


  1. Fowler JF. What can we expect from dose escalation using proton beams. Clin Oncol. 2003;15(1):S10-S15.
  2. Zeitman AL, DeSilvio ML, Slater JD, et al. Comparison of conventional-dose vs high-dose conformal radiation therapy in clinically localized adenocarcinoma of the prostate [published correction appears in JAMA. 2008;299(8):898-899]. JAMA. 2005;294(10):1233-1239.
  3. Steneker M, Lomax A, Schneider U. Intensity modulated photon and proton therapy for the treatment of head and neck tumors. Radiother Oncol. 2006;80(2):263-267.
  4. Miralbell R, Lomax A, Cella L, Scheider U. Potential reduction of the incidence of radiation-induced second cancers by using proton beams in the treatment of pediatric tumors. Int J Radiat Oncol Biol Phys. 2002;54(3):824-829.
  5. Chung CS, Keating N, Yock T, Tarbell N. Comparative analysis of second malignancy risk in patients treated with proton therapy versus conventional photon therapy. Int J Radiat Oncol Biol Phys. 2008;72(1):S8.
  6. Lee CT, Bilton SD, Famiglietti RM, et al. Treatment planning with protons for pediatric retinoblasta, medulloblastoma, and pelvic sarcoma: how do protons compare with other conformal techniques? Int J Radiat Oncol Biol Phys. 2005;63(2):362-372.
  7. Komaki R, Sejpal S, Wei X, et al. Reduction of bone marrow suppression for patients with stage III NSCLC treated by proton and chemotherapy compared with IMRT and chemotherapy. Particle Therapy Cooperative Group 47. 2008;O10:14.
  8. Mayahara H, Murakami M, Kagawa K, et al. Acute morbidity of proton therapy for prostate cancer: the Hyogo Ion Beam Medical Center experience. Int J Radiat Oncol Biol Phys. 2007;69(2):434-443.
  9. National Association for Proton Therapy Web site. Accessed September 15, 2010.