We believe that cancers usually start with abnormal changes in just a single cell, or a very small number of cells, in a particular part of the body. Over a period of time these cells multiply, and eventually form a tumour.

Usually the earliest a breast cancer, or any other type of cancer, can be detected is when it is about 0.5cm (about 1/4 inch) across - and most are only discovered when they are several centimetres in diameter. By the time a cancer has reached a diameter of 0.5cm it will contain about a hundred million cells, and will have been growing for a long time, probably quite a few years.

Mammograms can first detect breast cancers when they are between about 0.5–1cm across, smaller tumours are very unlikely to show up on the x-rays. This means that although from a medical point of view we say that mammograms can detect breast cancer at any early stage, when it is still highly curable, these 'early' cancers will actually have been growing for a long time, but will have been completely undetectable by any of the tests that we have at the present time.

If someone has a normal mammogram, and then two years later, when the test is repeated, it shows up a small cancer in their breast, this doesn't mean that the tumour first started during the interval between the x-rays, but simply that it has grown to a size when it can be seen on the mammogram during that two year period - the cancer itself will almost certainly have first started some years before, but was simply too small to show up on the early x-ray films.

Incidentally, when a new breast cancer appears in the two years between screening tests, doctors often call these 'interval' cancers, since they have become detectable in the interval between mammograms.