Some of the highest breast cancer rates are found in the US, where breast cancer strikes one in every nine women, and 40,000 Americans die of it every year. The picture in the UK is only slightly better, with one in 12 women at risk, but a staggering 33,000 new cases are diagnosed annually - twice the rate of only 40 years ago.

The bare statistics seem frightening, and have been used by doctors to press-gang women into being tested for breast cancer as early as possible. In the US, screening for breast cancer has become a huge money-making industry - a trend echoed even in Britain’s cash-strapped NHS.

But what if the figures are wrong? What if medicine is seriously overdiagnosing as cancer a condition that is essentially harmless?

During the last 10 years, breast screening has been called into question largely over basic questions of accuracy.

In fact, a growing number of experts believe that the advent of breast-cancer screening has created a problem where none may actually exist, labelling and treating many conditions as cancer which aren’t serious or life-threatening.

The astonishing fact is that fully half of all cases of so-called ‘breast cancer’ might not be cancer at all, but a harmless abnormality that will never progress to cancer. In some 16,000 cases in the UK and 40,000 cases in the US, women could be being wrongly treated for cancer.

What is breast cancer? Breast cancer is a growth of undifferentiated cells in the breast area usually causing a lumpy tumour. However, the overwhelming majority - some 80 per cent of breast tumours - is not cancerous.

The breast is a fairly simple organ, mostly made up of fat, and lymphatic and connective tissues. Milk is produced in the nodules, and a system of ducts passes the milk to the nipple. It is in these lobules and ducts that cancer is believed to develop, eventually spreading out to the other parts of the breast and forming a tumour.

Although doctors often pretend otherwise, the various stages of breast cancer are still not well understood.

The first stage of one type of cancer is believed to be when a milk duct or lobule is invaded by microscopic calcifications. Most of these are so tiny that they cannot be seen or felt, and are only detectable on a mammogram. The calcifications are believed to be the precursors of cancer, but they are not in themselves cancerous. Nevertheless, they are somewhat misleadingly called ‘carcinomas in situ’ (CIS), which means ‘cancers in place’. Doctors refer to the calcifications that occur in lobules as ‘LCIS’ and the ones in ducts as ‘DCIS’, which is much the more common diagnosis of the two.

Before mammography, DCIS was virtually unknown, but it now accounts for up to 50 per cent of breast cancer diagnoses. The conventional view is that identifying DCIS is a good thing as it picks up cancer in the early stages, thus enabling treatment to prevent the cancer from developing.

At least, this is the message given to patients, but some experts are beginning to question the whole philosophy.

'Doctors should make it clear that DCIS is not cancer; it is only a possible precancer,' says Dr Eric Wiener, head of breast oncology at the Dana-Farber Cancer Institute in Boston, Massachusetts.