If the tumour can disseminate not only in the beginning of the tumours life but during the whole the life time, then earlier diagnosis will lead to a reduction in number of cancers with metastases. Long average lead-time will then be associated with a lower rate of cancers diagnosed with metastases (which may lead to a reduction in mortality).

The average lead-times in the randomized trials have been estimated to be about 4 years. However, these calculations are based on the assumption that

a: all tumours grow and

b: that the slow-growing tumours have very low sensitivity.

Multiple screening rounds will then empty the reservoir of slow-growing tumours and lead to a strong decline in incidence rates when women are not invited to screening any more. When the randomized screening trials were stopped, mammography screening started immediately in the control group. Therefore, these studies cannot be used to calculate incidence rates when women are not invited to mammography screening. The calculation of lead-time in the randomized trails is therefore very speculative because they have not observed the lead-time.