Abstract
The association between breast and endometrial cancer was investigated in a cohort consisting of 60,065 subjects (99% of all women in whom a first breast cancer was diagnosed in Sweden in 1960-63 and 1968-81). Complete follow-up until 1981 revealed a total of 260 endometrial cancers, as against an expected number of 151.1 (relative risk (RR) = 1.72; 95% confidence limits (CL) 1.46; 1.87). RR increased steadily from close to unity in women younger than 50 at breast cancer diagnosis to 2.40 (CL 1.97; 2.93) in those 70 years of age and older. The excess number of endometrial cancers occurred primarily during the first five years of follow-up (RR = 2.07; CL 1.79; 2.38). A common causal agency for breast and endometrial cancer is more likely to lie in environmental than in genetic factors and other observations in the same population do not support that such factors are related to characteristics of the women's reproductive histories.
Highlights
There was a regular trend towards an increase in relative risk, from a value close to unity (RR = 1.07; confidence limits (CL) 0.72-1.54) in women younger than 50 years at breast cancer diagnosis to 2.47 (CL 1.57-3.71) in those 80 years or older (Table II)
The increased risk was greatest in the first years after the breast cancer diagnosis
The highest risk was incurred during the first year after diagnosis by patients 70 years of age or older, with 37 observed versus 9.89 expected cases of endometrial cancer (RR=3.74; CL 2.64-5.16)
Summary
Since 1958, when the National Swedish Cancer Registry was started, all newly diagnosed malignant tumours have had to be reported to the Registry. This obligation rests upon both the physician and the pathologist or cytologist who confirms the diagnosis on surgically removed tissues, biopsies, cytologic specimens or at autopsy. The relative risk was defined as the ratio of observed numbers of cases of endometrial cancer to expected numbers. The 95% confidence limits (CL) of the relative risk were calculated on the assumption that the observed number of cases follows the Poisson distribution (Bailar & Ederer, 1964).
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