The study lead, Professor Judith Bliss said, "We're delighted that the results of this trial have the potential to lead to a real change in the way selected breast cancer patients are treated".
Professor Arnie Purushotham, from Cancer Research UK, said: "One of the challenges when treating early stage breast cancer is trying to minimise the side effects that can have a real impact on a woman's life, without affecting the chances of curing her".
The research shows that patients who receive radiotherapy directly on their original tumour site experience fewer side-effects than those who have the treatment over their entire breast.
The trial involved more than 2,000 women aged 50 or over in the United Kingdom with early stage breast cancer, that was at low risk of coming back.
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"This approach could spare many women significant physical discomfort and emotional distress".
A third then received whole breast radiotherapy - which is the current clinical standard - and a third received targeted breast radiotherapy.
"Now we have evidence to support the use of less, but equally effective, radiotherapy for selected patients".
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The technique can be carried out on standard radiotherapy machines, thus the study results will lead to further uptake of this treatment at centres worldwide, Bliss noted.
Hilary Stobart, 62, from Cambridgeshire, was treated with partial breast radiotherapy after being diagnosed with breast cancer in 2009.
Most women with breast cancer would usually receive radiation therapy to the full breast, but it can cause side effects such as hair loss, fatigue, aches and soreness.
"I was in the group that had the least amount of radiotherapy".
After presenting the results to colleagues, this method has been chosen to be considered for selected patients by the UK Royal College of Radiologists 2016 breast radiotherapy consensus and the Danish Breast Cancer Apology Group.
She said: "Any cancer diagnosis is shocking".
Unfortunately, if you've had breast cancer or breast surgery, you would not qualify to take part in the study.