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<strong>IARC</strong> MONOGRAPHS VOLUME 79
1942, were used to identify incident cases of cancer, including benign brain tumours <strong>and</strong>
bladder papillomas (Olsen et al., 1989), in the patients. The expected numbers of cases
were calculated from national cancer incidence rates, similarly based on the files of the
Cancer Registry. In two updated reports with extended follow-up (Clemmesen &
Hjalgrim-Jensen, 1977, 1981), the original roster was revised to include 8078 <strong>and</strong> 8077
subjects, respectively. The reduction of the cohort by 12% was due to exclusion of non-
Danish citizens, patients who died before 1943 when the follow-up began, duplicate
registrations <strong>and</strong> persons whose diagnosis of epilepsy was not sustained. In the most
recent update (Clemmesen & Hjalgrim-Jensen, 1981), the period of follow-up was from
4 weeks after first admission to the treatment community or 1 January 1943, whichever
came last, to the day of death or the end of 1976, whichever came first. A total of 467
cases of cancer were observed when 419.5 were expected [yielding an overall st<strong>and</strong>ardized
incidence ratio (SIR) of 1.1; 95% confidence interval (CI), 1.0–1.2; or 1.25
(95% CI, 1.1–1.4) among men <strong>and</strong> 0.99 (95% CI, 0.87–1.13) among women]. The
overall increase was due mainly to the observation of 71 tumours of the brain <strong>and</strong>
nervous system when 13.4 were expected [yielding a significantly increased SIR of 5.3;
5.8 among men <strong>and</strong> 4.8 among women]. Excess rates of brain tumour were seen in
particular during the first 15 years of follow-up [with SIRs of 12, 5.4, 1.8, 2.7, 1.4, 2.1
<strong>and</strong> 0.0 for latency periods of 0–9, 10–14, 15–19, 20–24, 25–29, 30–34 <strong>and</strong> ≥ 35 years,
respectively]. The authors suggested that the seizures of some patients were early
symptoms of their brain tumours. Thirteen cases of primary liver cancer were observed,
when 3.44 cases were expected [SIR, 3.8; 95% CI, 2.0–6.5]. This excess of liver cancer,
which was particularly evident in patients followed for 15 years or more, was ascribed
to use of radioactive thorium dioxide (Thorotrast) for cerebral angiography in a
subgroup of patients during diagnostic work-up. Only three of the 13 patients with
primary liver cancer had had no documented exposure to Thorotrast, which was in
agreement with the 3.4 cases expected among all cohort members. Finally, the increased
SIR for lung cancer among male patients [SIR, 1.3; 95% CI, 0.94–1.7] was considered
most likely to be due to a higher prevalence of smokers.
The Danish cohort was further evaluated by Olsen et al. (1989), who extended
follow-up for cancer incidence from 1976 through 1984. Exclusion of additional duplicate
admissions, non-Danish citizens <strong>and</strong> patients for whom the data were incomplete
<strong>and</strong> the inclusion of patients hospitalized for fewer than 4 weeks reduced the cohort to
8004 patients. Of these, 140 had had documented exposure to Thorotrast. Linkage of
the records of the remaining 7864 patients not known to have received this contrast
medium with the files of the national Cancer Registry resulted in identification of 789
cancers, with 663.7 expected (SIR, 1.2; 95% CI, 1.1–1.3). A sixfold increased risk was
seen for brain cancer on the basis of 118 observed cases; 43 were seen within 1 year of
admission (SIR, 88). The risk for brain tumours in childhood was especially high. A
significant excess also occurred for cancer of the lung, but with no clear trend over
time. Non-significantly increased risks were seen for cancers of the liver (SIR, 1.9) <strong>and</strong>
biliary tract (SIR, 1.7) on the basis of 9 <strong>and</strong> 11 observed cases, respectively. It was