Women in Britain with breast cancer could benefit from a technique reducing some side effects of surgery.
Breast cancer is the most common form of the disease for UK women
Currently, when a doctor thinks the cancer could spread, glands under the arm are removed, as well as the tumour.
But research co-ordinated by Cardiff scientists suggests it is unnecessary in two-thirds of cases and they have developed a less drastic treatment.
The procedure could be more widely available throughout the UK within 18 months.
Removing the lymph nodes from under the arm is a painful procedure which can result in loss of movement.
Research carried out over three years across England and Wales found that by removing just one gland from under the arm, specialists can tell whether the cancer has spread.
Using a small dose of radioactivity, doctors are able to locate the main gland - called the sentinel node - that drains directly from the tumour.
The Department of Health will fund a training programme for doctors to learn the technique.
Professor Robert Mansel, professor of surgery at the University of Wales College of Medicine, Cardiff, said: "The standard practice at the moment for managing the lymph glands under the arm is that we have to take them all out.
"The reason for that is we need to know whether the cancer has spread to them.
"The new technique allows us just to take the most important node - the sentinel node.
"This means the woman who doesn't have any spread only has a very small operation."
Claire Claridge lost mobility in her arm after having glands removed
Professor Mansel is presenting the findings of the trials to the American Society of Clinical Oncology on Saturday.
Claire Claridge was one of 1,000 women who took part in the research team's trials of the new techniques.
She was one of the 500 who had the standard procedure and has lost some mobility in her right arm as a result of the operation.
She said: "It means you can't carry heavy things with that arm even if you've got the full movement back.
"I have a daughter and if it helps her generation it will please me.
"I know it can make a lot of difference to women in the near future as well."
In the UK, the lifetime risk for breast cancer in women is one in nine.
It is the most common cancer affecting women in the UK, with more than 38,000 new cases each year.
Delyth Morgan, of Breakthrough Breast Cancer, said: "Women we talk to are very concerned to know whether their breast cancer has spread or not but the removal of lymph nodes can sometimes be extremely debilitating.
"Any technique that is less invasive and provides an accurate answer will be welcomed by women with breast cancer.
"It's important to remember that at the moment, sentinel node biopsy is not yet standard treatment.
"However, we look forward to finding out the results of this trial which, if positive, will help inform National Institute of Clinical Excellence guidance and as a result, make this technique available to all women who could benefit."