Multiple cancer--an epidemiologic exercise in Finland.

The material of the Finnish Cancer Registry from 1953-79 (279,745 cancer patients, 774,518 person-yr at risk) was analyzed for the occurrence of multiple cancer. There were 5,871 new primary cancers in the series (excluding the first 12 mo from diagnosis of the first cancer). A positive association between cancers with similar etiology could be demonstrated, e.g., between cancers of the lip, larynx, and lung (smoking) and between cancers of the breast and endometrium (hormones, reproductive history). Clustering of different risk factors resulted, e.g., in an excess risk of colon cancer among female breast cancer patients (risk factors in both cancers are prevalent particularly in higher-social classes). Differences in the distribution of the risk factors resulted in risk deficits, e.g., low relative risk of lung cancer among male colon cancer patients (the prevalence of smoking was highest in the lower-social classes and the prevalence of risk factors in colon cancer was highest in the higher-social classes). The risk of leukemia was increased among patients with cancers of the breast, endometrium, and thyroid (possibly due to irradiation). There was a high relative risk of salivary gland cancer among patients with skin cancer other than melanoma or basal cell carcinoma in both sexes. The relative risk of a new primary cancer decreased with increasing age at diagnosis of the first cancer. The length of follow-up was positively associated with the relative risk of many cancers, although this finding was not as consistent as that with age.