The studies, published in The Lancet and The Lancet Oncology, found tumours were less likely to return in patients diagnosed with the disease at an early stage who received radiotherapy during or immediately after surgery, than those who received it later.
Most women who undergo partial mastectomy for breast cancer also have a course of radiotherapy on the whole of the affected breast - which has been shown to cut recurrence risk.
However, radiotherapy can cause unpleasant side-effects, and requires women to attend hospital for 20 to 30 days after surgery.
In some cases, women eligible for partial mastectomy or lumpectomy end up undergoing full mastectomy, as they cannot meet the demands of subsequent radiotherapy treatment.
In The Lancet, researchers led by Professor Michael Baum and Professor Jayant Vaidya, of University College London, investigated the performance of TARGIT, where radiation is delivered to the tumour site via a tiny X-ray emitting device, compared with a standard course of daily doses for three to six weeks.
They found that when TARGIT was given at the same time as lumpectomy, the recurrence of the tumour and breast cancer deaths were similar, and in fact deaths from other causes were significantly lower with TARGIT.
Professor Vaidya said: "The most important benefit of TARGIT for a woman with breast cancer is that it allows her to complete her entire local treatment at the time of her operation, with lower toxicity to the breast, the heart and other organs."
A similar conclusion was reached in the study published in The Lancet Oncology. Researchers in Italy, led by Professor Umberto Veronesi, of the European Institute of Oncology in Milan, reported results from a trial of a slightly different method of delivering targeted radiation, using a device which emits electron radiation to the tumour site, electron intraoperative radiotherapy or ELIOT.
Breast cancer is the most common cancer. In Scotland, one in eight women will be diagnosed with the disease in their lifetime. More than 80 per cent of women with breast cancer are still alive five years after diagnosis.
James Jopling, director for Scotland at Breakthrough Breast Cancer, said: "Radiotherapy is a vital part of breast cancer treatment as it reduces the risk of breast cancer recurring. "However, we know from speaking to patients how difficult and inconvenient daily treatments can be, so the possibility of receiving radiotherapy in one dose is very encouraging."