Looking backwards from outcome to exposure to assess cancer latency

Assessing time relationships in cancer epidemiology involves linking exposure and cancer incidence or mortality on a time scale. The traditional approach is to make some exposure criterion the referent point, such as date of first exposure. However, the time relationships between exposure and cancer may also be usefully explored using the date of diagnosis (or death) as the referent point in time. This has been accomplished to date in industry-based case-control studies by estimating the etiologic fraction among ever exposed cases for years prior to death from cancer, and in the serially additive expected dose (SAED) model by estimating excess case exposure dose for each year prior to cancer death. Each of these measures assesses exposure in time windows (of arbitrary width) prior to cancer death or diagnosis. Both measures are mathematically related (by constants) to a measure of the fraction of excess case exposure dose achieved in each time window, where "excess" means beyond that expected on the basis of non-cases or controls. This particular measure, the retrospective excess exposure fraction (REEF) in time windows, may be valuable in exploring time relationships between exposure and cancer. It is concluded that retrospective measures between cancer occurrence and exposure warrant further attention to maximize the informativeness of cancer epidemiology studies.