DIETHYLSTILBESTROL AND RISK OF FATAL BREAST-CANCER IN A PROSPECTIVE COHORT OF US WOMEN

The authors examined the association between the use of diethylstilbes trol during pregnancy and the risk of subsequent fatal breast cancer i n a large prospective study of US adults. After 9 years of follow-up, 1,574 cases of fatal breast cancer were observed among 501,536 gravid women who reported no prior history of cancer in 1982. Results from Co x proportional hazards models showed a positive association between a history of diethylstilbestrol exposure (reported by 3.9% of all women) and fatal breast cancer (adjusted rate ratio = 1.34, 95% confidence i nterval 1.06-1.69). This excess risk did not increase over time; women who were exposed more than 35 years ago (rate ratio = 1.35, 95% confi dence interval 0.97-1.87) were not at greater risk than women who were exposed within the past 35 years (rate ratio = 1.39, 95% confidence i nterval 1.01-1.93). The positive association was not observed in women who used diethylstilbestrol before age 25 years but was seen at all o ther ages. The age of study participants did not modify the associatio n between exposure and breast cancer, and there were no significant in teractions between ever use of diethylstilbestrol and any of the other potential risk factors included in the analysis. These findings are c onsistent with those of several other studies of diethylstilbestrol ex posure and breast cancer.