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Interval cancers are cancers discovered between two screening campaigns.

The diagram below attempts to explain how such interval cancers occur. Let us suppose that the cancer can be clinically discovered when its diameter is around 18 mm (blue horizontal line).

If the campaign screens the population every two years (red numbers), the screening test will reveal the cancer between test T2 and test T3 (i.e. a tumour which has just attained a diameter of 18 mm).

If the campaign screens the population every three years (blue numbers), the T2 test might easily fail to spot the tumour which measures only 15 mm in diameter, and it will only be discovered at T3 (i.e. when the cancer is already larger than 2 cm in diameter).

It is almost impossible to totally avoid the occurrence of interval cancers:

In well elaborated screening policy, these interval cancers should be of a small size: they reflect the degree of uncertainty of the intervals between screening campaigns, which are adjusted to the common speed of growth of the cancer (or the pre-cancer states) and to the cost and the discomfort of the tests proposed.

Independently of good policy, there will always be very aggressive cancers which do not follow the usual biology of their type of cancer or do not respond to usual therapies.

Unfortunately, these interval cancers may also be a sign of mistakes in the interpretation of screening tests: a highly secure reading of the test should be provided (for instance by double reading mammographies or pap smears).