Correspondence
The Risk of Breast Cancer after Giving Birth
N Engl J Med 1995; 332:63-64January 5, 1995
- Article
To the Editor:
Lambe et al. (July 7 issue)1 present strong data in support of a transient increase in the risk of breast cancer after giving birth. The authors conclude that the transient increase is more pronounced for women who are older at the time of delivery than for those who are younger.
The authors bolster this claim with a small P value (<0.001) reported for the interaction between age at the time of delivery and age at the time of the diagnosis of breast cancer and with an illustration of the corresponding predictions from their model in their Table 2. However, the cited interaction is equivalent to an interaction between age at delivery and time since delivery, because the age at diagnosis and the time since delivery are, for a given age at delivery, just different ways of saying the same thing. The reported interaction, then, simply recapitulates the overall pattern of a decrease in the odds ratio with increasing time since delivery, irrespective of the age at delivery. The test that the authors conducted is an inappropriate basis for concluding that the transient increase in risk is higher for women who deliver at an older age.
1 ReferencesW. Douglas Thompson, Ph.D.
University of Southern Maine, Portland, ME 041031
Lambe M , Hsieh C-c,Trichopoulos D ,Ekbom A ,Pavia M ,Adami H-O . Transient increase in the risk of breast cancer after giving birth. N Engl J Med 1994;331:5-9
Full Text | Web of Science | Medline
Author/Editor Response
The authors reply:
To the Editor: We indicated that to examine whether there is a transient increase in the risk of breast cancer after childbirth, age could be considered as an effect modifier. Since the total effect of childbirth is at issue, the P value we reported applies to the odds ratio for the comparison of uniparous women of a specific age at delivery with nulliparous women, as a function of age at the time of the diagnosis. The observation that the increase in risk was more pronounced among women who were older at the time of delivery was not based on this test, as Dr. Thompson suggests, but was inferred from the estimates in Table 2 and Figure 1 of our article.
To address Dr. Thompson's question, one might need to perform more complicated analyses that allowed the modifying effect of age on the risk associated with childbirth to be divided into two parts: the interaction with parity and the interaction with age at delivery. Examination of this and other aspects of the observed short-term effect of childbirth requires a much larger data set, and this approach is currently being explored.
Mats Lambe, M.D.
University Hospital, S-751 Uppsala, SwedenChung-cheng Hsieh, D.Sc.
Harvard School of Public Health, Boston, MA 02115
