BBC Correspondent Josh Bassett assesses the significance of these findings

They compared standard tests with a more recently developed technique, and found that the old methods missed hidden tumours picked up by the newer one.

When women have a breast cancer removed, pathologists check their lymph nodes - a central part of the immune system found in the armpit - to see if any of the tumour has got into the blood stream.

If it has, the cancer may recur or spread to another part of the body, so doctors use radiation or chemotherapy to destroy remaining cancer cells.

But the study, published in The Lancet medical journal, found that current tests missed many hidden tumours and their use was "no longer clinically tenable".

Immune response

An international team of researchers found that standard methods detected potential tumours in seven per cent of the group.

Once the disease is detected, doctors must decide the treatment

The new method stains the tissue with a chemical that triggers an immune-system response to highlight anything that should not be there.

It found potential tumours in 20% of the group, which was studied over a 12-year-period. The researchers also found that those with hidden tumours were more likely to be revisited by cancer.

By highlighting the hidden tumours, doctors can decide which patients will benefit most from secondary treatments such as chemotherapy and radiation treatment.

'Reassured falsely'

Professor Gordon McVie, director of the Cancer Research Campaign, said it was a well-conducted large-scale study that supported what many people had claimed for a long time.

"The study shows it's time pathologists caught up with modern technology. If one test means they miss a certain proportion of cancers then they should use the other one.

"Patients are being reassured falsely - they're being told 'you're in a low-risk group because your armpits are clear' and they're not."

'Good case for new tests'

Dr Andrew Hanby, a specialist in breast cancer pathology at the Imperial Cancer Research Fund, is investigating the usefulness of different methods of testing.

He said the researchers made a good case for changing to the newer tests.

"Nobody with breast cancer is given the all-clear," he said.