The rate of the astronauts has also been compared with the rate of the LSAH comparison participants.
All cancer deaths among the LSAH population have been among males; therefore only males were included in these analyses. This is a conservative analysis approach since the denominator used in the rate calculation is based on the total population being examined. Given a consistent numerator (the number of cancer cases) the smaller denominator obtained by including only males results in a higher rate. All males selected as astronauts between 1959 and 1995 (210) and male comparisons for the same time period selected from the JSC employee population (618), as participants in LSAH were included in this cancer mortality investigation. Person years of follow-up time for each of the two study groups were calculated for the same age-specific groupings identified in the reference population of Public Health Region 6. The expected rates of cancer mortality are obtained by applying the age-specific rates of the reference population to the study population. The results are those rates that would be expected if the study population had experienced the same cancer mortality as the reference population. The observed rates of cancer mortality in each of the two LSAH study groups were compared with the expected rates.
The ratio of the observed rate to the expected rate is known as the Standardized Mortality Ratio (SMR). An SMR of 100 indicates that identical rates exist for two groups. Confidence intervals (CI) around the SMRs (CI, 95%) were calculated.
Both the astronauts and the comparison group currently have lower age-specific rates of total cancer mortality than that seen in the general population residing in the Texas Gulf Coast area. The rate among the astronauts is somewhat higher than the comparison participants but not significantly higher. The difference may be from chance alone.
There have been three cancer deaths among the astronaut population. The SMR of 47 indicates that the observed cancer deaths among the astronaut population are 53 percent less than the 6.25 expected deaths. The confidence intervals indicate that, even though the observed rate of cancer mortality is noticeably lower than expected, it is not statistically different from the general population.
There have also been three cancer mortalities among the LSAH comparison population versus an expected 17.3 cases. The resulting SMR of 17 (CI = 3.5, 37.9) is significantly lower than expected.
The particular types of cancers experienced among the LSAH population are relatively rare among the general population. Conversely, the more common cancers among the general population (lung, colorectal, prostate) have not been identified as causes of mortality among the LSAH population.
Comparisons made between astronaut health data and general population health data should be viewed with some skepticism because of the differences between the astronauts and the general public. Risk factors associated with many cancers are also associated with life-styles and education. The astronauts are highly educated and receive routine preventive medical care. They are, in general, physically fit and nonsmokers. Because of the difficulties associated with making comparisons between the astronauts and the general population, the comparison population was selected from the JSC employees. Age, sex, and body mass index were used as matching criteria, and all JSC employees have access to preventive medical care through the Occupational Medicine Clinic. A comparison of cancer rates between these two groups is much more statistically sound than comparisons made with the general public.
If the astronauts experienced the same age-specific rates of cancer as the comparison participants, 0.87 deaths would have been expected compared with the three observed deaths. This results in an SMR of 345 (CI = 69.5 to 756.2). The rate of total cancer mortality among the astronauts is elevated when compared with the LSAH comparison participants, but not significantly different.
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