What are the side effects of proton therapy?

Proton therapy is radiation treatment. Radiation damages cancer cells, and it also affects healthy tissue in and around the treatment area. So proton therapy causes side effects.

A proton beam stops at the tumour and gives almost no dose beyond it. A proton plan therefore puts less total radiation into the body than an X-ray plan. That lowers the dose in some places. It does not remove side effects.

What follows is what published studies report. What to expect in your own case is a question for the team that will treat you.

What this page covers

Why side effects depend most on the area being treated.
What is reported during treatment and shortly after.
What is reported months and years later.
Where the evidence is still uncertain.

What causes side effects in radiation treatment?

Side effects come from the dose that reaches healthy tissue near the tumour. They depend most on which part of the body is treated, how much dose is given, and the age of the person treated.

This is why there is no single list of proton therapy side effects. Treatment to the head produces different effects than treatment to the chest. Canada’s health technology review of proton therapy found that its safety varied by the type of cancer, with more harm recorded in some cancers and less in others.

What are the short-term side effects?

Short-term effects appear during treatment or in the weeks after it. Most settle over time.

Head and neck. Mouth soreness, difficulty swallowing, dry mouth, taste changes, weight loss, and a skin reaction where the beam enters. In a randomised trial of 440 patients, severe swallowing difficulty affected 34 percent of the proton group and severe dry mouth 33 percent. Severe skin reaction affected 24 percent of the proton group, which was higher than the 18 percent recorded in the group treated with X-rays.

Brain. Fatigue, hair loss in the path of the beam, and headache. In a study of 120 adults treated for brain tumours with pencil-beam scanning, 83.5 percent had mild or moderate short-term effects and none had a severe one. Quality of life dipped after treatment and improved again from one year onward.

Chest and esophagus. Painful swallowing and inflammation of the lung. In a randomised lung cancer trial, severe lung inflammation occurred in 10.5 percent of the proton group and 6.5 percent of the group treated with X-rays.

Prostate and pelvis. Bowel urgency, more frequent bowel movements, and urinary symptoms. In a 2025 study that was not randomised, bowel function worsened in a way the patients themselves rated as meaningful in 26 percent of those treated with protons and 47 percent of those treated with X-rays. Urinary symptoms did not differ.

Breast. Skin reaction. In a study of 8,189 patients treated in routine care, skin reaction of any grade was recorded in 82.0 percent of those treated with protons and 57.1 percent of those treated with X-rays. Moderate skin reaction was less common after protons, at 13.4 percent compared with 20.9 percent.

What are the long-term side effects?

Long-term effects can appear months or years after treatment ends.

Radiation necrosis is damage to tissue in the treated area. In the brain it is the effect most watched for. In 163 adults treated for cancers at the base of the skull, moderate or worse damage to the brainstem occurred in 1.3 percent within five years, and every affected patient recovered with medical treatment. A review of 11 studies covering 2,210 children found brainstem effects in 1.8 percent, and death from that cause in 0.5 percent. The risk is small. It is not zero.

Other long-term effects depend on the area treated. They can include hearing loss, hormone problems, and changes in thinking and memory when the brain is in the treatment field.

What are the long-term effects in children?

Children are treated with proton therapy so that less radiation reaches growing tissue. Published follow-up shows that this reduces some late effects without removing them.

In a study of 59 children treated for medulloblastoma with proton therapy, followed for a median of seven years:

55 percent had a hormone problem within five years, most often growth hormone deficiency.
12 percent had severe hearing loss within three years.
Full-scale IQ fell by an average of 1.5 points per year.
No heart, lung, or digestive late effects were recorded.

Survival matched what is achieved with standard radiation, and the authors described the level of toxicity as acceptable. A 2025 review of non-cancer effects after proton therapy in children reached the same conclusion in plainer words: the risk is reduced after proton therapy, but it is not eliminated.

Does proton therapy reduce the risk of a second cancer?

Radiation can cause a new cancer years later. Because a proton beam puts less total radiation into the body, a lower risk is expected. The published evidence does not yet confirm it.

A review of 24 studies covering 38,163 children treated for tumours of the brain and spinal cord found second cancers in 1.5 percent after proton therapy and 1.8 percent after X-ray therapy. That difference was not statistically significant. The authors noted that follow-up after proton therapy is shorter, and that second cancers often take more than a decade to appear.

So the expectation is reasonable and the proof is not yet in. A claim that proton therapy prevents second cancers goes beyond what has been shown.

What is still uncertain?

Treatment planning assumes that a proton beam is 1.1 times as biologically damaging as an X-ray beam, and treats that figure as fixed. Published work shows it is not fixed, and that it appears to rise near the point where the beam stops. What this means for healthy tissue in real patients is not settled. One study looked for a link between this effect and brain damage after treatment and did not find one. A 2025 position paper from NRG Oncology, a clinical trials group in the United States, sets out the question and calls for further work.

This is an active research question rather than a reason to avoid treatment. It appears here because an accurate account of proton therapy includes what is not yet known.

Frequently asked questions

Does the treatment itself hurt?

No. The beam is not felt while it is delivered. Side effects build up over a course of treatment rather than arriving during a single session.

Are proton therapy side effects milder than standard radiation?

It depends on the area treated. In esophageal and head and neck cancer, randomised trials recorded fewer severe effects with proton therapy. In lung and breast, some effects were recorded more often with proton therapy. The trial results are set out on proton therapy vs IMRT. IMRT, short for intensity-modulated radiation therapy, is the standard X-ray radiation used in Canada.

How long do side effects last?

Short-term effects usually settle within weeks to a few months after treatment ends. Long-term effects can appear later and some are permanent. The treating team can say what to watch for in a given treatment plan.

Who can tell me my own risk?

The physician who will deliver the treatment. Side effect risk depends on the diagnosis, the exact treatment plan, and the person’s own health. No website can estimate it.

Sources for this page (13)

Safety varied by the type of cancer treated: Kim J, Wells C, Khangura S, et al., Proton Beam Therapy for the Treatment of Cancer in Children and Adults: A Health Technology Assessment, CADTH Health Technology Assessment No. 145, Ottawa, August 2017. ncbi.nlm.nih.gov (checked 2026-08-16)
Head and neck, severe swallowing difficulty, dry mouth and skin reaction rates: Frank SJ, Busse PM, Lee JJ, et al., Proton versus photon radiotherapy for patients with oropharyngeal cancer in the USA: a multicentre, randomised, open-label, non-inferiority phase 3 trial, Lancet 2026;407(10524):174-184, doi:10.1016/S0140-6736(25)01962-2, PMID 41391462. doi.org (checked 2026-08-16)
Brain, short-term effects and quality of life in 120 adults treated with pencil-beam scanning: Kroeze SGC, Mackeprang P, De Angelis C, et al., A Prospective Study on Health-Related Quality of Life and Patient-Reported Outcomes in Adult Brain Tumor Patients Treated with Pencil Beam Scanning Proton Therapy, Cancers 2021;13(19):4892, doi:10.3390/cancers13194892, PMID 34638375. doi.org (checked 2026-08-16)
Lung, severe lung inflammation rates: Liao Z, Lee JJ, Komaki R, et al., Bayesian Adaptive Randomization Trial of Passive Scattering Proton Therapy and Intensity-Modulated Photon Radiotherapy for Locally Advanced Non-Small-Cell Lung Cancer, J Clin Oncol 2018;36(18):1813-1822, doi:10.1200/JCO.2017.74.0720, PMID 29293386. doi.org (checked 2026-08-16)
Prostate and pelvis, bowel and urinary outcomes in a study that was not randomised: Gergelis KR, Bai M, Ma J, et al., Long-Term Patient-Reported Bowel and Urinary Quality of Life in Patients Treated with Intensity-Modulated Radiotherapy Versus Intensity-Modulated Proton Therapy for Localized Prostate Cancer, Curr Oncol 2025;32(4):212, doi:10.3390/curroncol32040212. doi.org (checked 2026-08-16)
Breast, skin reaction rates in 8,189 patients treated in routine care: Yu CC, Hsieh CC, Huang YT, et al., Toxicity profiles of proton and photon radiotherapy in postoperative breast cancer: a 10-year real-world cohort study, Breast Cancer Res 2026;28:49, doi:10.1186/s13058-026-02227-6. doi.org (checked 2026-08-16)
Brainstem damage in adults treated for cancers at the base of the skull: Holtzman AL, Rutenberg MS, De Leo AN, et al., The incidence of brainstem toxicity following high-dose conformal proton therapy for adult skull-base malignancies, Acta Oncol 2022;61(8):1026-1031, doi:10.1080/0284186X.2022.2101900. medicaljournalssweden.se (checked 2026-08-16)
Brainstem effects and deaths in children, pooled from 11 studies: Almalki SF, et al., Brainstem Toxicity Following Proton Beam Radiation Therapy in Pediatric Brain Tumors: A Systematic Review and Meta-Analysis, Cancers 2024;16(21):3655, doi:10.3390/cancers16213655, PMID 39518092. doi.org (checked 2026-08-16)
Long-term effects in children treated for medulloblastoma, hormone, hearing and IQ outcomes: Yock TI, Yeap BY, Ebb DH, et al., Long-term toxic effects of proton radiotherapy for paediatric medulloblastoma: a phase 2 single-arm study, Lancet Oncol 2016;17(3):287-298, doi:10.1016/S1470-2045(15)00167-9, PMID 26830377. doi.org (checked 2026-08-16)
The risk is reduced after proton therapy but not eliminated: Zajac-Grabiec A, Biesaga B, Krzyzowska M, et al., Non-cancer effects after proton beam therapy for pediatric tumors, a narrative review, Front Oncol 2025;15:1554765, doi:10.3389/fonc.2025.1554765, PMID 40519295. doi.org (checked 2026-08-16)
Second cancer rates in children, and the shorter follow-up after proton therapy: Palmer JD, Upadhyay R, Yadav D, et al., Risk of secondary malignant neoplasms in children following proton therapy vs photon therapy for primary CNS tumors: A systematic review and meta-analysis, Front Oncol 2022;12:893855, doi:10.3389/fonc.2022.893855. doi.org (checked 2026-08-16)
The assumed biological effect of a proton beam is not fixed and varies near the end of its range: Paganetti H, Simone CB 2nd, Bosch WR, et al., NRG Oncology White Paper on the Relative Biological Effectiveness in Proton Therapy, Int J Radiat Oncol Biol Phys 2025;121(1):202-217, doi:10.1016/j.ijrobp.2024.07.2152, PMID 39059509. doi.org (checked 2026-08-16)
No link found between that effect and brain damage after treatment: Niemierko A, Schuemann J, Niyazi M, et al., Brain Necrosis in Adult Patients After Proton Therapy: Is There Evidence for Dependency on Linear Energy Transfer?, Int J Radiat Oncol Biol Phys 2021;109(1):109-119, doi:10.1016/j.ijrobp.2020.08.058, PMID 32911019. doi.org (checked 2026-08-16)

Every statement on this page is drawn from the sources listed below. Last updated: 16 August 2026.

This page is for general information only. It is not medical advice and it is not a decision about your care or your funding. Only your treating physician can advise you on treatment. Only your provincial or territorial health plan can decide whether it will fund treatment outside the country. protontherapy.ca is an information resource by Maple Med Global (MMG Medical Tourism Inc.), Toronto, Canada. We are not a hospital, a clinic, or a government body, and we do not provide medical care.