Problems in classification of cancer for epidemiologic research.

In vital statistics and most epidemiologic studies, cancers have been classified mostly by site of origin alone. This continues to be true even though it is continually being demonstrated that among cancers of a site important subsets with different epidemiologies almost always are present. Reasons for epidemiologists' failure to use all the information contained in the standard cancer classification are explored as are problems that arise from the nature of the classification, from the nature of the cancers being classified, and even from patient characteristics that determine how much information on the cancer can be gathered. The solution to the problem of too little information is generally difficult, but pathologists can say more about the epidemiologic implications of their various diagnoses and epidemiologists can learn to use these diagnoses in their cohort and other studies.