Intended for healthcare professionals

Practice Uncertainties

Should patients requiring radiotherapy for breast cancer be treated with proton beam therapy?

BMJ 2023; 381 doi: (Published 09 June 2023) Cite this as: BMJ 2023;381:e072896
  1. Anna M Kirby, consultant clinical oncologist1,
  2. Francesca Holt, NHS clinical oncology specialist trainee and NIHR doctoral research fellow2,
  3. Carolyn W Taylor, professor of oncology and honorary consultant oncologist2,
  4. Joanne S Haviland, reader in medical statistics3,
  5. Mairead MacKenzie, patient advocate4,
  6. Charlotte E Coles, professor of breast cancer clinical oncology and NIHR research professor5
  1. 1Royal Marsden NHS Foundation Trust & Institute of Cancer Research, Sutton, UK
  2. 2Nuffield Department of Population Health, University of Oxford, UK
  3. 3Centre for Evaluation and Methods, Wolfson Institute of Population Health, Queen Mary University of London, UK
  4. 4Independent Cancer Patients’ Voice, UK (patient author)
  5. 5Department of Oncology, University of Cambridge, UK
  1. Correspondence to A Kirby Anna.Kirby{at}

What you need to know

  • Proton beam therapy is a type of radiation therapy that uses protons (high energy charged particles) rather than standard x ray radiotherapy, to treat cancer

  • There are no randomised trial data supporting the routine use of proton beam therapy in patients with breast cancer, however the treatment can be more accurately targeted than x rays, potentially reducing risks of side effects in organs such as the heart

  • A small cohort of breast cancer patients has a higher-than-average lifetime risk of radiation induced heart disease, and may benefit from proton beam therapy

More than 2.3 million new cases of breast cancer were diagnosed worldwide in 2020, according to the World Health Organization. In the UK, around 56 000 people receive a diagnosis of breast cancer each year, of which around two thirds receive radiotherapy.1

Current standard care for breast cancer is x ray radiotherapy, which reduces the risk of relapse by a third to a half and, in many patients, improves survival with few long term side effects.2 For example, in patients with cancer identified in the lymph nodes, radiotherapy reduced absolute breast cancer mortality by 6% at 20 years.3 In breast cancer radiotherapy, radiation dose is delivered to the target areas, ie, the breast or chest wall +/- regional lymph nodes (depending on the patient’s situation) using x ray beams. An international standard radiation dose in this setting is 40 Gy in 15 fractions delivered over three weeks, with the START trial having shown that this regimen is as effective as the previous standard of care of 50 Gy in 25 fractions over five weeks, but with lower risks of side effects.4

One of the limitations of x ray radiotherapy in patients with breast cancer is that it also delivers small amounts of radiation to surrounding organs, …

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