Occupation and cancer - European Trade Union Institute (ETUI)

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with the deficits seen for prostate cancer, stomach
cancer, myeloma and leukaemia.
The small inaccuracies in cancer registration are
not likely to affect the SIR estimates of the present
study because they are not related to occupation. The
accuracy of the diagnostic procedure, instead, may
vary over educational and socio-economic strata, and
there may even be occupation-specific special features
such as annual health controls that directly
affect the likelihood of a non-symptomatic cancer
diagnosis of the employee. Most of the categories
with the highest and lowest incidence of unspecified
cancers in the present study (Tables 78, 79) were also
among the top and bottom risk occupations of all
cancers combined (Tables 80, 81). Hence, the likelihood
of getting a proper diagnosis for the cancer
seems not to vary markedly between occupations.
Accuracy of linkage and person-year calculation
The linkage between the census data, the mortality
and emigration data and the cancer incidence data
was based on the unique personal identity codes
used in all five countries. Apart from errors in the
identifiers, which are extremely rare, the method
thus by definition ensured a complete ascertainment
of relevant events [158].
If the follow-up for vital status would be incomplete,
there would be a risk of bias related to the
occupational variation in general mortality. Fortunately
the Nordic population register systems offer
very accurate data on the vital status of all residents,
and therefore the person-year calculations in the
present study are precise.
Statistical significance
We present in this study SIRs for men and women,
five countries, 54 occupational categories and close
to 80 diagnostic groups, i.e., about 40 000 SIRs.
Inevitably, many of these combinations will by
chance come out with significantly high or low
SIRs. In the interpretation of the findings, it is
therefore important to pay attention not only to the
size of the SIRs or the confidence intervals, but also
to the consistency across countries and the biological
plausibility.
The present study covers up to 45 years of cancer
incidence in the population born between 1896 and
1960 and living in the five Nordic countries;
Denmark, Finland, Iceland, Norway and Sweden.
Almost 15 million persons were included in the
study, and close to 385 million person years at risk
were accumulated. Approximately 3 million incident
cancer cases occurred during the follow-up
Occupation and cancer in Nordic countries 771
period. The present study is thus the largest study
ever reported on occupational cancer incidence.
Due to the huge size of the study, many of the
observations that are statistically significant correspond
to such a small deviation from unity that it
has no practical implication. For instance, a great
majority of the 54 occupation-specific SIRs for
all sites combined were statistically significant (42
SIRs among the men and 40 among the women),
although the relative risk difference in some instances
was not more than some 2% as compared
with the reference rate.
Selection of reference rates
Selection of the reference population has an effect on
the SIR values. In the present paper, the expected
numbers of cancer cases were based on national
incidence rates. The other easily available option as
to the reference rates would have been to use the
rates calculated for the entire Nordic population.
SIRs based on national rates and on Nordic incidence
are dissimilar for cancers with large variation
in incidence levels between the countries. In testicular
cancer the incidence rate in Denmark during
the follow-up period of the present study was about
twofold, in Finland only one-half of the Nordic
average (Figure 34). The SIR for testicular cancer
among Finnish seamen based on the national
reference rate was 2.29 (95%CI 1.18 4.00, 12
cases), but only 1.06 (0.55 1.86) if based on the
combined Nordic rate (Table 83). In turn, the SIR
for Danish farmers increases from 0.84 (0.71 0.98,
155 cases) to 1.75 (1.49 2.04) if Nordic rates are
used instead of Danish rates. In comparison to the
Nordic reference rates, virtually all occupationspecific
SIRs in Denmark are statistically significantly
increased and most SIRs in Finland are
statistically significantly decreased.
There are several factors affecting the general
national incidence rates, including differences in
the prevalence of aetiological factors, diagnostic
procedures and even registration practices. These
factors are hardly related to direct occupational
hazards, and therefore we decided that in a study
aiming at describing relative risks related to occupation,
it is best justified to compare the incidence of
cancer in a given occupation in a given country with
the general population in the same country.
There is also variation in the background cancer
incidence within countries (Figure 50, [167]). If a
given occupation is concentrated in specific regions,
then the national reference rate may not be fully
appropriate. Previous studies (e.g., Pukkala et al.,
1997 [168]), however indicate that the use of