Women fighting breast cancer could be given less radiotherapy after research showed larger doses given less often were just as effective.
Experts found raising the dosage size produced similar side effects to the standard treatment, such as breast hardness, swelling and pain, but led to fewer skin changes.
Two trials by the Institute of Cancer Research (ICR) sought to discover if the international standard of 50 grays delivered in 25 fractions (treatments) could be lowered without affecting the chances of recovery.
Women in trial A had 39 or 41.6 grays in 13 fractions within five weeks, while trial B patients received 40 grays in 15 fractions during three weeks.
Both sets of results published in The Lancet Oncology journal showed breast cancer tumours were kept under similar control to women having a standard course of radiotherapy treatment.
The study, which was part of the Start trials involving 4,451 patients, was funded by Cancer Research UK, the Department of Health and the Medical Research Council.
It questioned more than 2,000 women who had surgery and radiotherapy for early-stage breast cancer, with 40% reporting marked or moderate changes in their breast during the following five years.
A total of 41% said they developed breast hardness, 39% reported a change in breast appearance, and nearly a third suffered arm and shoulder pain, which was linked to earlier treatments.
The ICR's chief investigator, Professor John Yarnold, said: "Using women's personal ratings of the side effects they experienced since treatment for early breast cancer gives us a unique insight into the effects of the lower dose of radiotherapy compared to the international standard."