Are Breast Cancer Rates Higher Because the Cape's Population is Older than the General Population of Massachusetts?

Breast cancer is more common in older women than in younger women. Cape Cod is home to many retirees, and the Cape's population is older on the whole than the population of the rest of Massachusetts. However, this difference does not explain the excess breast cancer incidence on the Cape. In calculating SIRs, we correct for age differences -- that's what "standardized" means in Standardized Incidence Ratio.

Remember that an SIR is a ratio, or fraction, with the actual number of cases in the numerator and the expected number in the denominator:

SIR = actual number of cases / expected number of cases

In calculating the expected number of breast cancer cases, we look at each age group separately, precisely because cancer rates are different at different ages.

The following table shows how the expected number of cases is calculated for a town on Cape Cod. We first calculate the breast cancer rate for each age group in non-Cape Massachusetts, our comparison area. We then estimate the number of women in each age group in the town. Next, for each age group, we multiply the rate in the comparison area by the population in the town to get the number of expected cases in that age group. Finally, we add up the number of expected cases in all the age groups. With the number of expected cases calculated in this way, the SIR reflects not only different cancer rates in different age groups, but also the age makeup of the town.

Does More Intensive Use of Breast Cancer Screening, Particularly Mammography, Result in More Women on the Cape Being Diagnosed?

At this time, no one has collected the data needed to study this question directly. Analyses of available data yield mixed results. Although there may be some effect of more intensive use of breast cancer screening in recent years, it is unlikely that it explains the elevation for the period as a whole.

Are Cape Women at Greater Risk Because of Individual Risk Factors such as Family History or Childbearing Patterns?

Silent Spring Institute asked researchers at Harvard University to address this question by reanalyzing data from an ongoing study, the Collaborative Breast Cancer Study. The Collaborative Breast Cancer Study is a large case-control study of breast cancer that includes women from both Cape Cod and non-Cape Massachusetts. For that study, researchers gathered extensive information on individual women, some with breast cancer and some without.

These researchers calculated that among women aged 50-74, women living on Cape Cod have a 21% higher risk of breast cancer than women elsewhere in Massachusetts, after taking account of differences in individual risk factors between the two groups. This analysis took account of numerous individual characteristics, including age, number of children, age at birth of first child, age at menarche and menopause, history of benign breast disease, education, and alcohol consumption. This finding suggests that the excess breast cancer incidence on Cape Cod does not occur simply because of differences in individual risk factors between Cape Cod and the rest of the state.

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