The CARET asbestos-exposed cohort: baseline characteristics and comparison to other asbestos-exposed cohorts

Our plain-English summary of The CARET asbestos-exposed cohort: baseline characteristics and comparison to other asbestos-exposed cohorts, by S. Barnhart, J. Keogh, M.R. Cullen, et al (American Journal of Industrial Medicine, 1997).

The study

• Authors: S. Barnhart, J. Keogh, M.R. Cullen, C. Brodkin, D. Liu, G. Goodman, B. Valanis, A. Glass, M. Thornquist, L. Rosenstock, G. Omenn, J. Balmes
• Journal: American Journal of Industrial Medicine
• Published: 1997;32(6):573-81
• Cited by: 25 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The Carotene and Retinol Efficacy Trial (CARET) was a double-blind, placebo-controlled trial of the daily administration of 25,000 IU vitamin A and 30 mg beta-carotene for the prevention of lung cancer. Of close to 18,500 participants, more than 4,000 were asbestos-exposed men recruited from shipyard and construction trades at five study centers in the United States. While the primary endpoint of the trial was the incidence of lung cancer, a number of questions about the natural history of asbestos-related disease will also be addressed. The mean age at entry into the trial was 57 years and the mean duration of follow-up on active intervention was 4 years. With the exception of 133 never-smoker pilot participants (3%), all subjects recruited were by intention current (38%) or ex-smokers (58%), with a mean cumulative smoking exposure at entry of 43 pack-years. Mean years from first asbestos exposure were 35, and mean duration of asbestos exposure in a high-risk trade was 19 years. The distribution of radiographic abnormalities was as follows: normal, 34%; parenchymal opacities (ILO profusion score > 1/0) alone, 18%; pleural thickening alone, 27%; both parenchymal opacities and pleural thickening, 21%. The CARET cohort, when compared to previously reported asbestos-exposed cohorts, is characterized by substantial asbestos exposure and high proportion of asbestos-related radiographic findings. The active intervention was halted in 1996, after a mean duration of 40 years. Passive follow-up of the cohort will continue until the year 2000.

In plain English

This article described the 4,060 asbestos-exposed men in CARET, a placebo-controlled trial of vitamin A and beta-carotene to prevent lung cancer, as they were when they entered the trial. The men were recruited through study centers in Baltimore, New Haven, Portland (Oregon), San Francisco and Seattle. Nearly all were current smokers (38%) or former smokers (58%), and smokers averaged 43 pack-years. On average, 35 years had passed since first asbestos exposure, and the men had spent 19 years in a high-risk trade. Plumbers/pipefitters, boilermakers and sheetmetal workers were the most common trades. On chest X-rays, 34% showed no asbestos-related abnormality, 18% lung-tissue (parenchymal) changes alone, 27% pleural changes alone, and 21% both. Overall, 39% had an ILO profusion score of 1/0 or greater. Of 3,661 men with adequate breathing tests, 40% had normal results. Lower breathing values were correlated with higher profusion scores. The authors concluded that, compared with earlier asbestos-exposed groups, this cohort had substantial exposure and a high proportion of asbestos-related X-ray findings. They noted it likely overestimates how common such findings are in these trades.

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: Description of the baseline (entry) characteristics of the asbestos-exposed cohort within a double-blind, placebo-controlled lung cancer chemoprevention trial (CARET), with comparison to previously reported asbestos-exposed cohorts
• Who was studied: 4,060 asbestos-exposed men recruited by CARET study centers in Baltimore, New Haven, Portland (OR), San Francisco and Seattle. They included 816 from a pilot study (1985 to 1988) and the rest from the full trial, which began in 1989. Mean age at entry was 57 years.
• Exposure: Occupational asbestos exposure beginning at least 15 years earlier. Eligibility required at least 5 years in a high-risk trade (asbestos insulators, sheetmetal workers, plumbers/pipefitters, plasterboard workers, boilermakers, shipyard electricians, ship scalers, ship fitters), or an X-ray showing asbestos-related changes together with substantial occupational exposure. Fiber types were not specified. Smoking was also recorded.
• Main results:
  • Smoking: 38% of the whole cohort were current smokers. Mean pack-years was 43 among former and current smokers.
  • Asbestos exposure: mean years since first exposure was 35, and mean years in a high-risk trade was 19.
  • The most common trades were plumbers/pipefitters (25%), boilermakers (19%) and sheetmetal workers (14%).
  • Chest X-rays across the cohort: 34% without an asbestos-related abnormality, 18% parenchymal abnormalities alone, 27% pleural abnormalities alone, 21% both.
  • 39% of participants had an ILO profusion score of 1/0 or greater.
  • Among the 3,661 participants (90%) with adequate spirometry: 40% had normal pulmonary function, 32% an obstructive defect, 13% a restrictive defect, 12% a mixed defect, and 2% small airways obstruction alone.
  • Reductions in percentage predicted FEV1 and FVC were correlated with ILO profusion scores.
  • Respiratory symptoms: cough was reported by 28%, phlegm by 40%, wheezing by 24%, and any dyspnea by 57% overall.
  • On a sample of 48 pilot films, exact agreement on profusion between the Seattle consensus readers and single readers in Baltimore, New Haven and San Francisco was 50%, 44% and 45%.
• The authors' conclusion: The authors concluded that, compared with other asbestos-exposed cohorts, the CARET cohort is characterized by substantial asbestos exposure, measured by time since first exposure, and by a high proportion of asbestos-related radiographic findings.
• Limitations the authors state:
  • Possible respondent bias in how the extent of asbestos exposure was classified, among men who volunteered for a prevention trial.
  • The cohort is not a random cross-section of exposed workers; it was drawn mainly from high-exposure trades.
  • Because of selection by latency and duration of exposure, the cohort likely overestimates the prevalence of asbestos-related X-ray abnormalities in these trades.
  • Nearly all participants were current or recent smokers. Together with the narrow range of asbestos exposure, this may limit assessment of dose-dependent effects of lower doses.

Topics

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

Medical terms in this study, explained in our Lexicon: Lung Cancer, Asbestos Exposure, Pleural Thickening.

Related studies

• Recruitment for the beta-carotene and retinol efficacy trial (CARET) to prevent lung cancer in smokers and asbestos-exposed workers — G.S. Omenn, G. Goodman, et al. (1992)
• Lobe of origin and histologic type of lung cancer associated with asbestos exposure in the Carotene and Retinol Efficacy Trial (CARET) — C.A. Brodkin, J. McCullough, et al. (1997)
• Risk factors for lung cancer and for intervention effects in CARET, the Beta-Carotene and Retinol Efficacy Trial — G.S. Omenn, G.E. Goodman, et al. (1996)
• CARET, the beta-carotene and retinol efficacy trial to prevent lung cancer in asbestos-exposed workers and in smokers — G.S. Omenn, G. Goodman, et al. (1991)
• Stopping the active intervention: CARET — D.J. Bowen, M. Thornquist, et al. (2003)
• Research cost analyses to aid in decision making in the conduct of a large prevention trial, CARET. Carotene and Retinol Efficacy Trial — M.D. Thornquist, N. Urban, et al. (1993)

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