Health Consultation Assessment of Drinking Water Quality And Cancer Incidence Scituate, ma


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A. Methods for Analyzing Cancer Incidence Data

Cancer incidence data were obtained from the Massachusetts Cancer Registry (MCR), of the MDPH, Bureau of Health Statistics, Research, and Evaluation, which has been monitoring cancer incidence in the state of Massachusetts since 1982. All newly diagnosed cancer cases are required by law to be reported to the MCR within six months of the date of diagnosis

(M.G.L. c. 11B). The 16-year period 1982-1997 constitutes the period for which the most recent and complete cancer incidence data were available at the time of this analysis. Cancer in general has a lengthy latency or development period that can range from 10 to 30 years and in some cases may be more than 40 or 50 years (Bang 1996, Frumpkin 1995). The latency period is the period between exposure to a disease causing agent and the appearance of manifestations of the disease (Last 1995). For the majority of tumors, the period between first exposure and the appearance of the tumor is 12 to 25 years. Therefore, this report summarizes cancer incidence data for the 11-year time period 1987-1997.

The MCR data are continually quality controlled so that corrections may be made in subsequent reports. Occasionally, the MCR research file may contain duplicate cases. For this analysis, each case of cancer was individually reviewed resulting in further refinement of the data that are included in the MCR City and Town Supplement Reports. In this evaluation, the MCR research file contained four duplicate cases of cancer.

To determine whether elevated numbers of cancer cases have occurred in the town of Scituate and each of its three CTs, cancer incidence data were analyzed by age and gender to compare the observed number of cancer cases to the number that would be expected based on the statewide cancer experience. Standardized incidence ratios (SIRs) were calculated for the


time period 1987-1997 for the five cancer types evaluated (i.e., breast, kidney, leukemia, liver, testes).

Specifically, an SIR is the ratio of the observed number of cancer cases to the expected number of cases multiplied by 100. An SIR of 100 indicates that the number of cancer cases observed in the population evaluated is equal to the number of cancer cases expected. An SIR greater than 100 indicates that more cancer cases occurred than expected and an SIR less than 100 indicates that fewer cancer cases occurred than expected. For example, an SIR of
150 is interpreted as the occurrence of 50% more cases than expected. Likewise, an SIR of
90 indicates the occurrence of 10% fewer cases than expected.

To determine if the observed number of cases is significantly different from the expected number, or if the difference may be due solely to chance, a 95% confidence interval (95% CI) was calculated for each SIR (Rothman and Boice 1982). A 95% CI assesses the magnitude and stability of an SIR. Specifically, a 95% CI is the range of estimated SIR values that have a 95% probability of including the true SIR for the population. If the 95% CI range does not include the value 100, then the study population is significantly different from the comparison or “normal” population. “Significantly different” means there is less than 5% chance that the observed difference is the result of random fluctuation in the number of observed cancer cases. When an SIR is statistically significant, as indicated by an * symbol, there is a less than 5% chance that the observed number of cases is due to chance alone. SIRs and 95% CIs are not calculated when the observed number of cases is less than five. A more detailed explanation of the SIR and 95% CI is provided in Appendix A.

The geographic distribution of cancer cases in Scituate was determined using available address information from the MCR indicating residence at diagnosis. This information was mapped for each individual using a computerized geographic information system (GIS)

(ArcView 1996). This allowed for the assignment of CT location for each case as well as an evaluation of the spatial distribution of cases at a smaller geographic level (i.e., neighborhoods). The geographic distribution was assessed using a qualitative evaluation of the point pattern of cases within the town. In instances where the address information was


incomplete (i.e., did not include specific streets or street numbers), efforts were made to research those cases using town residential lists issued within two years of an individual’s diagnosis and site visits to the area.

The MCR routinely collects data related to risk factors for individual cancer cases (e.g., age, occupation). The available risk factor information from the MCR was evaluated for breast cancer, testicular cancer, leukemia, kidney cancer, and liver cancer. However, information about personal risk factors (e.g., family history, hormonal events, and diet) which may also influence the development of cancer, is not collected by the MCR and, therefore, could not be evaluated in this investigation.

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