Such differences derived from the above-mentioned different composition for age and sex of examined population and probably from a different underlying incidence.
Although absolute numbers were higher due to a higher underlying incidence of breast cancer, the ratio of breast cancer deaths avoided per case overdiagnosed were equal to those observed in our selected cohort (Supplementary Table 4).
Including 5% tumours with 5 years clinical lead time increased the estimated clinical lead time by 0.14 years; assuming a 1% annual underlying incidence increase added only 0.01 year to the estimate; assuming a 50% higher incidence reduction after screening increased the estimate by 0.06 year, and combining all three extreme assumptions increased the estimate by 0.18 year.
This corresponds to an analysis of the underlying incidence of CIN3+ (annual rate of development of new cases) on the assumption that new cases remain detectable at least until the next smear.
The reasons for this widespread upward trend in developed countries are not fully understood, since it may be largely artefactual due to the progressive implementation of modern diagnostic imaging techniques over recent decades, or it may partly reflect a real increase in underlying incidence linked to changes in environmental exposures to lymphomagenic agents and other unknown risk factors.
Cancer patients in our study cohort were at significantly higher risk of a second diagnosis compared to the underlying incidence rates experienced in the entire population of Queensland.
If the average age at prevalence screen is 51 years, the appropriate estimated underlying incidence should be a weighted average of the 45 49 and 50 54 year rates, weighting the first by 0.3 and the latter by 0.7.
Comparison of interval cancer rates by unit may therefore have introduced potential confounders at a geographical level, such as variation in underlying incidence or the recording of interval cancers, or the possibility that units which were quicker to change to two-view mammography are more effective in identifying interval cancers a priori than those still using one view.
However, given the differences in the underlying incidence between cancer sites, the total number of avoidable incident cancer cases in men was largest for colon (676), followed by rectal (605), liver (508), oral cavity (330), pharyngeal (304) and laryngeal (276) cancer.
