Breast Cancer as a Risk Factor for Other Primary Malignant Diseases. A Nationwide Cohort Study2

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Abstract
A cohort comprising 11,452 women and based on virtually all cases of first incidence of breast cancer in Sweden in 1964–67 was followed for 13–16 completed years—corresponding to 94,078 person-years of observation—for the occurrence of subsequent primary tumors other than in the breast. A slight excess in the total number of second malignant diseases (695 observed vs. 633.7 expected; relative risk (RR) = 1.10) was possibly partly due to a higher autopsy rate with more frequent detection of preclinical cancers in the cohort than in the background population. A significant risk increase was confirmed for endometrial cancer after the age of 70 years (RR = 2.4; 95% confidence limits = 1.6–3.5) but not for ovarian cancer (RR = 1.2) or tumors of the large bowel (RR = 1.2), rectum (RR = 1.1), or any other site. It was concluded that: 1) so many subgroups were analyzed in this study that the significant association with endometrial cancer might be due to the play of chance alone and, therefore, needs further confirmation, 2) there is no common genetically linked susceptibility to primary cancer of the breast and that of other sites, 3) the hypothesis that cancers of the breast and large bowel share major etiologic factors was not supported, and 4) it is unlikely that secondary cancers significantly contribute to the late excess mortality in breast cancer patients.