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OCR for page 61
11
Findings
In this chapter, we present the findings from our standardized mortality ra-
tio and proportional hazards analyses.! We begin with tests of the primary study
endpoints and the presentation of some descriptive data. The findings from these
analyses suggested additional avenues of investigation, the results of which are
then reported.
TESTS OF PREDETERMINED PRINCIPAL ENDPOINTS
We had determined in advance of data collection that participant versus ref-
erent mortality rates would be formally compared for three endpoints all
causes, all malignancies, and all leukemia minus chronic lymphoid leukemia
(CLL). We had further decided to examine these outcomes using both standard-
ized mortality ratios (SMRs) and proportional hazards analyses. SMRs were
used to compare mortality rates for participant and referent subjects with U.S.
white male population rates.2 However, these SMRs take into account only age
and calendar year of death, and the mortality rates of the U.S. general population
are generally higher than those of military veterans. When a comparable referent
group is available, proportional hazards analyses allow for simultaneous control
of design variables (via stratification) as well as tighter control for age differ-
ences (via covariate adjustment) and thus provide a better basis for estimating
the difference in mortality experience of the two groups.
SMR data in Table 11-1 show that the SMRs for both all-cause mortality
and all-malignancy mortality were almost equal for participants and referents,
whereas participants had a higher SMR for leukemia death (0.75) than did refer
iSee Chapter 9 and Appendix C for explanation of these methods.
2See Chapter 9 for discussion of the use of white male rates.
61
OCR for page 62
62
THE FIVE SERIES STUDY
ents (0.651. As anticipated, all SMRs are less than 1.00, indicating that both the
participant and the referent cohorts had lower mortality than the general popula-
tion the "healthy veteran effect" (Seltzer and Jablon, 1974, 1977~. The propor-
tional hazards analyses also show with estimated hazard ratios (HRs~that
participants and referents were at similar risk of all-cause mortality (HR = 1.00)
and all-malignancy mortality (HR = 1.02), and participants had an estimated 14
percent higher risk of leukemia death (HR = 1.14~. However, none of these haz-
ard ratios is significantly different from 1.00, indicating that there were no sta-
tistically significant differences between the participant and referent cohorts on
these outcome measures.
DESCRIPTIVE ANALYSES OF PREDETER1VIINED
ADDITIONAL MORTALITY ENDPOINTS
We also looked at a number of mortality endpoints that had been examined
in other studies, particularly cancer endpoints. Table 11-2 shows SMRs for
broad categories of noncancer causes of death. Only one SMR was greater than
1.00 symptoms, signs, and ill-defined conditions. Proportional hazards analy-
sis shows only two significant differences. Participants had a significantly
higher risk of death due to external causes (1.08; 95% CI 1.02-1.16) and a sig-
nificantly lower risk of death due to unknown cause (0.62; 95% CI 0.57-0.67~.
The latter is not surprising because it reflects the fact that the causes were miss-
ing for 4.5 percent of participant deaths versus 7.3 percent of referent deaths, a
statistically significant difference (see Table 8-4 and related discussion). Partici-
pants also had higher risks of death due to diseases of the musculoskeletal sys-
tem (1.43; 95% CI 0.86-2.38) and congenital anomalies (1.59; 95% CI 0.72-
3.51~; however, these are based on relatively small numbers of deaths.
Table 11-3 shows data for various cancer mortality endpoints in some de-
tail. Again, almost all SMRs are less than 1.00. The six cancer sites with esti-
mated hazard ratios greater than 1.2 are nasal cancer (2.64; 95% CI 1.02-6.82),
thyroid cancer (2.33; 95% CI 0.83~.55), cancer of the testes (1.62; 95% CI
0.59-4.46), male breast cancer (1.39; 95% CI 0.53-3.66), bone cancer (1.21;
95% CI 0.57-2.60), and prostate cancer (1.20; 95% CI 1.03-1.40~. Only the
risks of death due to nasal cancer and prostate cancer were significantly higher
among participants than referents. Among the hematologic cancers, the risks for
all leukemia (1.15; 95% CI 0.93-1.43) and leukemia minus chronic lymphoid
leukemia (CLL) (1.14; 95% CI 0.90-1.44) were both elevated, albeit not signifi-
cantly, among participants.
Table 11-4 shows a more detailed breakdown of leukemia deaths by sub-
Wpe, as available from death certificates. The highest hazard ratios are associ-
ated with two types of acute leukemia: lymphoid leukemia excluding CLL (2.05;
95% CI 0.71-5.92) and myeloid leukemia excluding chronic myeloid leukemia
(1.44; 95% CI 1.00-2.09~.
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OCR for page 69
FINDINGS
69
INVESTIGATING LEUKEMIA RISK BY LAND AND
SEA SERIES PARTICIPATION
We stated earlier that although we had not identified any unambiguous proxy
measures for radiation dose, we had considered the possibility that the different
test series or the land and sea series represent differences in exposure experience.
Exposures were not uniformly distributed within series. Operation
PLUMBBOB, for example, consisted of 30 tests, including safety tests that pro-
duced negligible yields as well as detonations of up to 44 kt (Harris et al., 1981~.
A formal test of heterogeneity of leukemia minus CLL risks among the individ-
ual test series was not statistically significant (x2 = 7.191, 4 df,p = .13~; thus, we
removed the series-specific analyses from the core presentation of this report. To
be complete, however, risk estimates of individual test series, service branch,
and paygrade groupings are presented in Appendix E.
Our interest in the land-sea dichotomy was based on the possibility that the
exposure experience was qualitatively and perhaps quantitatively-different at
the two test sites. In Table 11-5, land series participation is defined as attendance
at UPSHOT-KNOTHOLE, PLUMBBOB, or any other test series conducted at the
Nevada Test Site, regardless of a participant's selection series. Similarly, atten-
dance at GREENHOUSE, CASTLE, REDWING, or any other series conducted at
the Pacific Proving Ground constituted sea series participation, regardless of se-
lection series. Members of the referent cohort were classified only by their coun-
terpart's selection series. Thus, because a participant may have attended both land
and sea tests, the number of land series participants plus sea series participants is
greater than the total number of participants. Approximately 5 percent of the par-
ticipant cohort members were associated with both land and sea senes.
TABLE 11-5. Number of Participants Who Participated in Any Land
Series and in Any Sea Series, by Selection Series
Selection Series
Selection Any Land Any Sea
Series Series Series
GREENHOUSE Sea 9,5287819,528
UPSHOT-KNOTHOLE Land 18,47318,473681
CASTLE Sea 15,68549015,685
REDWING Sea 12,9231,16012,923
PLUMBBOB Land 11,55911,559481
Total 68,16832,46339,298
Participants in both land and +2,431+1,162
sea senes
NOTE: Boldface signifies personnel counted in both "any land" and "any sea" cate-
gories.
No member of the Referent Cohort participated in any test series. To maintain com-
parability between cohorts, referents are assigned the selection series of the matched
participant unit.
OCR for page 70
70
THE FIVE SERIES STUDY
Table 11-6 shows that for all-cause mortality, land series participants have a
slightly, but significantly, lower risk relative to referents (HR = 0.96; 95% CI
0.93-0.99), whereas sea series participants have a slightly higher, also significant,
risk relative to sea series referents (HR = 1.03; 95% CI 1.00-1.06~. Neither land
series nor sea series participants had a significantly higher risk of all-malignancy
death (HR= 1.00 and 1.04, respectively). However, land series participants show a
statistically significant increase in the hazard ratio for death due to leukemia, 1.49
(1.0~2.13~; for sea series participants, the hazard ratio was 0.92 (0.67-1.27), not
significantly different from 1.00. Thus, participation in a land series is associated
with a significantly higher risk of leukemia death, while participation in a sea se-
ries is associated with a significantly increased all-cause death rate.
INVESTIGATING LEUKEMIA RISK BY TIME SINCE FIRST
PARTICIPATION AND AGE AT FIRST PARTICIPATION
To explore the increased, but not statistically significant, risk of leukemia
observed among participants relative to referents, we looked for patterns conso-
nant with other research findings on the association of radiation and leukemia.
We therefore fit a model using time-dependent covariates to estimate the risk of
leukemia death in three periods relative to first series participation: less than 5
years, 5 years to less than 15 years, and 15 years or more after participation. We
created a model to estimate the risk of leukemia mortality for three ranges of age
at first participation: less than 20 years of age, 20 to 25 years, and 25 years and
older. Even with this study's very large cohort, there were insufficient early leu-
kemia deaths to yield a definitive picture. Tables 11-7 and 11-8 (see page 72)
show that the relationship between participant status and leukemia mortality
does not seem to follow anticipated patterns of latency and age at exposure
(Boice, 1996~.
OCR for page 71
71
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OCR for page 72
72
THE FIVE SERIES STUDY
TABLE 11-7. Hazard Ratios for Leukemia,
Excluding Chronic Lymphoid Leukemia, by
Time After First Exposure
Hazard Ratio*
(95% CI)
Time After First
Exposure
<5 years
5 to <15 years
215 years
0.80 (0.2~2.42)
1.09 (0.59-2.02)
1.16 (0.90-1.49)
NOTE: CI = confidence interval.
*Participant cohort relative to referent cohort.
Proportional hazards model controls for series, service
branch, and paygrade by stratification and age, age
squared, and age cubed by covariate adjustment.
TABLE 11-8. Hazard Ratios for Leukemia,
Excluding Chronic Lymphoid Leukemia, by
Age at First Participation
First Participation Hazard Ratio*
Age (years) (95% CI)
<20
20to<25
225
1.18 (0.52-2.70)
1.22 (0.81-1.85)
1.08 (0.79-1.47)
NOTE: CI = confidence interval.
*Participant cohort relative to referent cohort.
Proportional hazards model controls for series,
service branch, and paygrade by stratification and
age, age squared, and age cubed by covariate
adjustment.
Representative terms from entire chapter:
series participants
