Comparison of cancer incidence and mortality in the Norwegian Fire Departments Cohort, 1960-2018

Our plain-English summary of Comparison of cancer incidence and mortality in the Norwegian Fire Departments Cohort, 1960-2018, by N. Marjerrison, J. Jakobsen, P.A. Demers, et al (Occupational and Environmental Medicine, 2022).

The study

• Authors: N. Marjerrison, J. Jakobsen, P.A. Demers, T.K. Grimsrud, J. Hansen, J.I. Martinsen, K.C. Nordby, M.B. Veierød, K. Kjærheim
• Journal: Occupational and Environmental Medicine
• Published: 2022;79(11):736-743
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: Elevated risk of cancer at several sites has been reported among firefighters, although with mixed findings. The purpose of this study was to calculate standardised incidence ratios (SIRs) and standardised mortality ratios (SMRs) for cancer and compare them to assess whether use of the different measures could be a source of inconsistencies in findings.

Methods: The Norwegian Fire Departments Cohort, comprising 4295 male employees who worked at 15 fire departments across Norway, was linked to health outcome registries for the period 1960-2018. SIRs and SMRs were derived using national reference rates.

Results: Overall, we observed elevated incidence of colon cancer (SIR, 95% CI 1.27, 1.01 to 1.58), mesothelioma (2.59, 1.12 to 5.11), prostate cancer (1.18, 1.03 to 1.34) and all sites combined (1.15, 1.08 to 1.23). Smaller, non-significant elevations were found for mortality of colon cancer (SMR, 95% CI 1.20, 0.84 to 1.67) and mesothelioma (1.66, 0.34 to 4.86), while SMR for prostate cancer was at unity. Potential errors were observed in some of the mortality data, notably for mesothelioma cases. Among those who died of cancer, 3.7% (n=14) did not have a prior diagnosis of malignancy at the same site group.

Conclusions: Assessment of incidence or mortality did not greatly influence the interpretation of results. The most prominent differences in SIR and SMR appeared to be due to inconsistencies between sites of cancer diagnosis and cause of death. The difference in SIR and SMR for prostate cancer suggested a detection bias from differential screening practices.

© Author(s) (or their employer(s)) 2022. Re-use permitted under CC BY. Published by BMJ.

In plain English

This study followed 4295 men from 15 Norwegian fire departments through national registries, 1960 to 2018. It compared their cancer diagnoses (incidence) and cancer deaths (mortality) with rates for all Norwegian men. Asbestos exposure was not measured. Mesothelioma diagnoses were higher than expected (8 cases; standardised incidence ratio 2.59, 95% CI 1.12 to 5.11). Mesothelioma deaths showed a smaller, non-significant elevation (ratio 1.66, 95% CI 0.34 to 4.86); only three were coded as mesothelioma. After the authors recoded two deaths recorded as cancer of the peritoneum and asbestosis, the death ratio was 2.77 (95% CI 0.90 to 6.47); they urged caution because reference rates could not be corrected the same way. Diagnoses of all cancers combined, colon cancer and prostate cancer were also above expected. Lung cancer incidence and mortality were close to expected. The authors concluded that using incidence or mortality did not greatly change the interpretation. They said the largest differences appeared to be related to mismatches between diagnosis and recorded cause of death, and that the prostate pattern suggested screening-related detection bias. They had no smoking or lifestyle data.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Retrospective cohort study linked to national cancer and cause-of-death registries, comparing standardised incidence ratios (SIRs) and standardised mortality ratios (SMRs) against national rates for Norwegian men
• Who was studied: 4295 male employees of 15 Norwegian fire departments, followed from 1 January 1960 to 31 December 2018. A subgroup of 3881 had held active firefighting positions. Women were excluded because of low numbers.
• Exposure: Employment at fire departments, including active firefighters as well as chimney sweeps, fire inspectors and office personnel. No asbestos or other exposure measurements and no fiber types were reported.
• Main results:
  • Full cohort, all cancers combined: SIR 1.15 (95% CI 1.08 to 1.23); SMR 1.08 (95% CI 0.98 to 1.20)
  • Full cohort, mesothelioma incidence: 8 cases, SIR 2.59 (95% CI 1.12 to 5.11)
  • Full cohort, mesothelioma mortality as originally coded: SMR 1.66 (95% CI 0.34 to 4.86). The 8 incident cases were followed by only three deaths coded as mesothelioma.
  • Four other deaths among men diagnosed with mesothelioma occurred within about 2 years of diagnosis. These were registered as cancer of the peritoneum, asbestosis, leukaemia or unknown/unattended. After two of these (peritoneum and asbestosis) were recoded to mesothelioma, the SMR was 2.77 (95% CI 0.90 to 6.47).
  • Full cohort, colon cancer: SIR 1.27 (95% CI 1.01 to 1.58); SMR 1.20 (95% CI 0.84 to 1.67)
  • Full cohort, prostate cancer: SIR 1.18 (95% CI 1.03 to 1.34); SMR 1.01 (95% CI 0.76 to 1.31)
  • Full cohort, lung cancer: incidence (SIR 1.02, 95% CI 0.82 to 1.25, per table 2) and mortality (SMR 0.92, 95% CI 0.71 to 1.17) were close to expected levels
  • Of the 376 men whose death was attributed to cancer, 14 (3.7%) had no prior cancer diagnosis at the same site or site group
• The authors' conclusion: The authors concluded that using incidence or mortality did not greatly influence the interpretation of results. They said the most prominent differences between SIR and SMR appeared to be related to mismatches between site of cancer diagnosis and cause of death. They added that mortality data can reveal biases in incidence, such as possible screening-related detection bias for prostate cancer.
• Limitations the authors state:
  • Cause-of-death data may contain misclassifications; death certificates are rarely validated against clinical or pathological information
  • The recoded mesothelioma mortality findings should be interpreted with caution, because similar corrections cannot be made to the reference rates
  • The cohort's size limited the power to detect small elevations in rare cancers
  • No data on lifestyle factors (such as smoking) were available
  • A healthy worker effect may bias estimates of occupational risk downwards

Topics

Medical terms in this study, explained in our Lexicon: Standardized Incidence Ratio (SIR), Colorectal Cancer, Mesothelioma.

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