Cancer registration in Denmark and the study of multiple primary cancers, 1943-80

The Danish Cancer Registry began in 1942 as the world's first program to register all cases of cancer arising in an entire nation. The Registry covers a population with free access to good medical care. Voluntary notifications are received of patients with reportable malignant and certain related diseases from hospital departments, pathology institutes, and practicing physicians. The Registry is linked annually to death certificates made available by the Danish National Board of Health to ascertain additional cancers and to learn whether patients previously reported to the registry have died. During the period 1943-77, coding of the reported diseases was done by the Registry's medical and clerical staff according to an extended version of the Seventh Revision of the International Classification of Diseases (ICD). Since 1978, information has been coded according to the ICD for Oncology. Multiple primary cancers in the same patient are entered individually into the Registry; however, before 1978 only multiple primary cancers in different organs were registered. An evaluation of the completeness and the validity of diagnoses in the Danish Cancer Registry generally confirms the high quality of its data. However, it appears that the approach taken by the Registry in accepting multiple primary cancers has been a conservative one, and the risk of a person developing second cancers of some sites will thus be underestimated.