Patterns of Breast Cancer Incidence on Cape Cod - the Standardized Incidence Ratio, or SIR

As part of the Cape Cod Breast Cancer and Environment Study, we examined breast cancer incidence on Cape Cod for the period 1982 through 1994. In all, 2,614 women diagnosed with breast cancer on Cape Cod during this period were included in the study.

To describe patterns of breast cancer incidence, we used the Standardized Incidence Ratio, or SIR, a common tool for monitoring disease rates.

Incidence is the number of newly diagnosed cases in a given location during a given time period. An SIR compares the actual number of cases for a given place and time to the number that would be expected based on cancer rates in some comparison area. In this study, we compared breast cancer incidence on Cape Cod to incidence in the rest of Massachusetts.

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

SIRs are usually written as 100 or 125 instead of 1.00 or 1.25. An SIR of 100 means that the actual number of cases equals the expected number. An SIR of 125 means that the actual number of cases was 25% higher than expected. An SIR of 75 means that the actual number of cases was only 75% of the expected number.

For the 13-year period of this study, the SIR for Cape Cod as a whole was 120 -- that is, breast cancer incidence on the Cape was 20% higher than in the rest of Massachusetts. This SIR is statistically significant, which tells us that this result is unlikely to have occurred by chance. Although breast cancer risk is not high in every town, elevated incidence occurs in towns all across the Cape (see map of SIRs for Cape Cod towns ). Excess breast cancer incidence is statistically significant in eight of the 15 towns on the Cape -- the probability is only one in 20 that these results occurred by chance.

To get a look at changes over time, we calculated SIRs for a series of short, overlapping time periods. This technique smoothes out fluctuations from year to year, which often are not meaningful. In this study, we calculated an SIR for 1982 through 1994, an SIR for 1983 through 1985, and so on for each three-year period through 1994. A line graph of SIRs by overlapping three-year periods is shown above in blue.

Breast cancer incidence on the Cape was most elevated during the early 1980s, became somewhat less elevated in the mid-1980s, and rose again in the late 1980s.

Institute researchers also examined patterns of mortality among women with breast cancer on Cape Cod using the Standardized Mortality Ratio (SMR) which is calculated in analogous fashion to the SIR.

For the period 1982-1992, Cape-wide mortality from breast cancer was neither significantly higher nor lower than expected based on death rates in the rest of the state.

Click here to see a graph of Breast Cancer Mortality for Cape Cod.

The data summarized here reflect the work done for the Cape Cod Breast Cancer and Environment Study. However, the Massachusetts Cancer Registry is continuously collecting new data and periodically issues reports. For the most up-to-date SIRs and SMRs for all cancers traced in Massachusetts, including breast cancer, contact MDPH at 617-624-5757 or visit their website at www.state.ma.us/dph/

People often ask three questions about elevated breast cancer incidence on Cape Cod:

Are breast cancer rates higher because the Cape's population is older than the general population of Massachusetts?

Does more intensive use of breast cancer screening, particularly mammography, result in more women on the Cape being diagnosed?

Are Cape women at greater risk because of individual risk factors such as family history or childbearing patterns?