The mean ages at diagnosis among BRCA2-positive cases and BRCA2-negative cases were 48.4 years (range 30-77 years) and 49.4 years (range 28-79 years), respectively. The mean ages when the BRCA2-positive cases, BRCA2-negative cases and controls had contributed information in the cohort study were 42.8 years (range 21-69 years), 43.3 years (range 20-69 years) and 42.8 years (range 21-69 years), respectively. The women were all born between 1912 and 1963 (median year of birth was 1939), and the cases were diagnosed in the years 1966-2001.
Table 1 compares the three groups of participants with respect to distribution of age at menarche and reproductive variables. Also presented are percentages with recorded responses for each item. The response rates are similar for the three groups. They are lowest for duration of breast feeding, due to the fact that registration of this variable first started in 1979. The BRCA2-positive cases were more likely than the controls to have given five or more births, whereas the BRCA2-negative cases had fewer births than the controls, as expected. The average total duration of breast feeding was lowest in the mutation carriers and was highest in the control group. The results did not allow for inferences with respect to whether this could be explained by problems with breast feeding in the mutation carriers. A very short total duration (less than 4 weeks) or no breast feeding was less common among parous BRCA2-positive cases than among noncarriers and controls, with percentages of 3.4%, 8.1% and 8.8%, respectively. The shorter average total duration of breast feeding in mutation carriers than in controls was mainly due to a lower percentage with overall duration of breast feeding for all children exceeding 2 years. When considering breast feeding for each child (data not shown), the percentages with duration of 4 weeks or less were similar in the mutation carriers and in controls (20.7%, 16.4% and 21.7% for BRCA2-positive cases, noncarriers and controls, respectively). However, BRCA2-positive cases were less likely than controls to have breast fed each child for longer than 6 months, with percentages being 5.2%, 12.0% and 11.9% for the affected mutation carriers, for the cases without the mutation and for the controls, respectively.
Table 2 presents the results from the multivariate analysis for parous women. The effects of age at menarche and of age at first birth were in the same direction for women with and without the mutation, and interaction with the mutation status was not statistically significant for these variables. On the contrary, each additional birth was associated with a nonsignificantly increased breast cancer risk (17%) among the mutation carriers, whereas a marginally significant 13% decrease in risk was observed for women without the mutation. Interaction was present between the mutation status and the number of births and between the mutation status and the total duration of breast feeding (P = 0.007 and P = 0.045, respectively). When limiting the analysis to cases diagnosed at age 40 years or older, the interaction was still present (P = 0.005 and P = 0.045 for the number of births and for the duration of breast feeding, respectively). Furthermore, after exclusion of 13 mutation carriers who had been recruited because of family history, the interaction remained statistically significant (P = 0.005 and P = 0.021 for the number of births and for the duration of breast feeding, respectively).
Breast Cancer Res. 2003;5(5) © 2003 BioMed Central, Ltd.
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