Use of multiple primary cancers to indicate associations between smoking and cancer incidence: an analysis of 500,000 cancer cases diagnosed in Norway during 1953-93.

The occurrence of multiple primary cancers is relatively rare, but may provide indications of common or opposite risk factors for different types of cancer. In the present study, the occurrence of multiple primary cancers was used to indicate possible associations between smoking and the incidence of cancers other than those generally accepted as smoking-associated. All cancer cases in persons above the age of 30, registered at the population-based Cancer Registry of Norway (1953-1993), were used in the analysis. For each type of cancer, the observed occurrence of smoking-associated cancers in the patients was compared with the expected occurrence if the patients had the same risk as the general population. Similar comparisons were made for the occurrence of other cancers in patients with a smoking-associated cancer. The results were presented as standardized incidence ratio (SIR), the ratio of the observed and the expected numbers of cases. The results indicated that uterine cervical cancer may share some important risk factor(s) with the cancers generally accepted as smoking-associated. This is in accordance with the literature, where an association between smoking and uterine cervical cancer has been found consistently. In addition, the results for liver cancer and leukemia indicated that these types of cancer also share some risk factor(s) with the smoking-associated cancers.