Incidence trends of mesothelioma in Norway, 1965-1999

Our plain-English summary of Incidence trends of mesothelioma in Norway, 1965-1999, by B. Ulvestad, K. Kjaerheim, B. Møller, et al (International Journal of Cancer, 2003).

The study

• Authors: B. Ulvestad, K. Kjaerheim, B. Møller, A. Andersen
• Journal: International Journal of Cancer
• Published: 2003;107(1):94-8
• Cited by: 25 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos exposure is considered to be the only important risk factor for malignant mesothelioma. The importation of asbestos to Norway increased after World War II and peaked in 1970. Stringent regulations took effect in 1977, and importation and use of asbestos practically ended in Norway in the late 1970s, until importation was prohibited in 1982. Our study aimed to analyze the incidence of mesothelioma in Norway according to temporal variation, to study the consequences of the use of asbestos and the asbestos ban effectiveness. An age-period-cohort model was used to analyze time trends for pleural mesotheliomas. From 1965-1999, the annual number of pleural mesotheliomas rose gradually both in males and females, and the highest annual number of pleural mesotheliomas was recorded in 1999 with 73 new cases diagnosed. The age-adjusted log linear drift of malignant mesothelioma of the pleura during the observation period rose 31.1% per 5 years among men and 15.9% among women. In 1995-1999, the age-adjusted incidence rate for men was 16.6 per million person-years for men and 2.3 for women. Cohort-specific risks increased for men born up to around 1935. After this the risks seem to stabilize. The rates were determined by age and by birth cohort. The delayed period effect of the asbestos regulation by the late 1970s will probably have its greatest effects on the mesothelioma rates around 2010.

Copyright 2003 Wiley-Liss, Inc.

In plain English

This study used Cancer Registry of Norway records to track mesothelioma in Norway from 1965 to 1999. Asbestos imports there peaked in 1970, strict rules took effect in 1977, and imports were banned in 1982. Pleural cases were analysed with an age-period-cohort model. Yearly pleural cases rose from 8 in 1965 to 62 in 1999 in men and from 3 to 11 in women. The age-adjusted trend rose 31.1% per 5 years among men and 15.9% among women. In 1995–1999 the age-adjusted rate was 16.6 per million person-years in men and 2.3 in women. Among men, risk rose for those born up to around 1935 and then seemed to stabilize; estimates for the youngest were very uncertain. Peritoneal mesothelioma rates stayed stable in men but rose in women, from 0.1 to 0.9 per million between the 1970s and 1990s. The authors concluded that pleural mesothelioma risk was still rising among Norwegian men, and that the late-1970s regulations would probably show their greatest effect around 2010. They added that judging the ban would take 10–20 more years.

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: Population-based cancer registry study of incidence trends over time, using an age-period-cohort model for pleural mesothelioma
• Who was studied: All mesothelioma cases of the pleura, peritoneum or other primary sites recorded in the Cancer Registry of Norway, 1965–1999. Table I lists 1028 pleural mesotheliomas in men and 170 in women. Table II lists peritoneal cases for 1970–1999 (39 in men, 31 in women). Mesotheliomas at other sites (17 men, 10 women) were excluded from the analyses.
• Exposure: No individual exposures were measured. The study related trends to national asbestos use: imports rose after World War II, peaked in 1970, were tightly regulated from 1977 and banned from 1982. Asbestos was mainly used for asbestos-cement products and insulation in ships and industry. The fiber types imported are not known; more than 70% came from Canada or the USSR, which exported mainly chrysotile, and some came from countries exporting amphibole asbestos.
• Main results:
  • The annual number of pleural mesothelioma cases rose from 8 in 1965 to 62 in 1999 in men, and from 3 in 1965 to 11 in 1999 in women. The highest annual total was 73 cases in 1999.
  • The age-adjusted trend (drift) in pleural mesothelioma rose 31.1% per 5 years among men and 15.9% per 5 years among women over 1965–1999.
  • In 1995–1999, the age-adjusted incidence rate of pleural mesothelioma was 16.6 per million person-years for men and 2.3 for women.
  • Among men older than 54, the annual number and rates of pleural mesothelioma kept increasing until 1999. Among men younger than 55, numbers rose until 1985–1989 and then stayed relatively stable.
  • Compared with men born around 1910, pleural mesothelioma risk increased for men born up to around 1935 and then seemed to stabilize. Wide confidence intervals left it uncertain whether risk had begun to fall in the youngest cohort.
  • Adding calendar-period effects to the model did not significantly improve its fit. In other words, the study found no statistically significant period effect such as one from the asbestos regulations.
  • Peritoneal mesothelioma rates among men stayed stable from 1970 to 1999. Among women they rose from 0.1 per million in the 1970s to 0.9 in the 1990s, and in the 1990s more peritoneal cases were recorded in women than in men (19 vs. 14).
• The authors' conclusion: The authors concluded that pleural mesothelioma risk is still increasing among men in Norway, with a lower but increasing trend in women. They expected the delayed effect of the late-1970s asbestos regulation to be greatest around 2010, and said a final conclusion on the ban's effectiveness could first be drawn in 10–20 years.
• Limitations the authors state:
  • The population had been followed for only about 20 years after the ban. The authors suggested this might be why no significant period effect was found.
  • Changes in diagnostic procedures may bias trend studies. Including very old people may cause underestimation because of poor investigation and competing diagnoses.
  • The fiber types imported to Norway are not known.
  • Peritoneal mesothelioma is difficult to diagnose and may be misclassified. For this reason only pleural cases were used in the age-period-cohort analysis.
  • Whether SV40 infection modified the trend could not be evaluated because the population is small and cases were few.
  • The authors found it problematic to forecast future case numbers from current cohort risks. They said such projections would likely be overestimates because they cannot account for future period effects of the regulations.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: Mesothelioma, Asbestos Exposure, Pleural Mesothelioma, Pleura, Incidence.

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