Mammograms are a very reliable way of detecting breast cancers but they are not foolproof. Around about one in ten breast cancers will either not show up at all on a mammogram or will look like a complete benign, non-cancerous, abnormality. These are called false negative results, since they suggest there is nothing to worry about, when in fact there really is a cancer in the breast.

For women who are attending a Breast Clinic after having found a breast lump the small chance that the mammogram is a false negative is less of a problem because the mammogram forms only one part of what is known as a 'triple assessment'. The triple assessment involves an examination of the breast by a specialist, a mammogram and a fine needle aspiration. (cells from the lump being removed by a special needle, with a local anaesthetic). The combination of these three techniques means that the correct diagnosis will almost always be made.

False negative mammograms are more of a problem in breast cancer screening, where women who have apparently normal breast have the test in order to detect small cancers with have not yet become apparent. If the mammogram looks normal then no further action is taken, so a false negative examination does mean that cancer can be missed. The chance of this happening has been reduced in recent years with the introduction of 'two-view' mammography into the UK screening programme in 1995. Before this screening often involved a single x-ray picture of each breast but it was then shown that taking two pictures from different positions increased the cancer detection rate by up to 25%. So, although it is more expensive and time consuming, the two view method has now become standard.

Although mammographic screening will still inevitably miss some early cancers it remains the best screening method there is and still detects very many unsuspected tumours every year when they are still at an early, curable, stage.