Pleural Plaques and the Risk of Lung Cancer in Asbestos-exposed Subjects

Rationale: Asbestos exposure is associated with a dose-dependent risk of lung cancer. The association between lung cancer and the presence of pleural plaques remains controversial.

Objectives: To define the relationship between pleural plaques and lung cancer risk.

Methods: Subjects were from two cohorts: 1) crocidolite mine and mill workers and Wittenoom Township residents and 2) a mixed-asbestos-fiber, mixed-occupation group. All subjects underwent annual review since 1990, chest X-ray or low-dose computed tomography scan, and outcome linkage to national cancer and mortality registry data. Cox regression, with adjustment for age (as the underlying matching time variable), was used to estimate hazard ratios (HRs) for lung cancer incidence by sex, tobacco smoking, asbestos exposure, presence of asbestosis, and pleural plaques.

Measurements and Main Results: For all 4,240 subjects, mean age at follow up was 65.4 years, 3,486 (82.0%) were male, 1,315 (31.0%) had pleural plaques, and 1,353 (32.0%) had radiographic asbestosis. Overall, 3,042 (71.7%) were ever-smokers with mean tobacco exposure of 33 pack-years. In total, 200 lung cancers were recorded. Risk of lung cancer increased with cumulative exposure to cigarettes, asbestos, and presence of asbestosis. Pleural plaques did not confer any additional lung cancer risk in either cohort (cohort 1: HR, 1.03; 95% confidence interval, 0.64-1.67; P = 0.89; cohort 2: HR, 0.75; 95% confidence interval, 0.45-1.25; P = 0.28).

Conclusions: The presence of pleural plaques on radiologic imaging does not confer additional increase in the risk of lung cancer. This result is consistent across two cohorts with differing asbestos fiber exposures and intensity.

Libby Amphibole Disease: Pulmonary Function and CT Abnormalities in Vermiculite Miners

OBJECTIVE

This article describes radiologic and pulmonary function findings among miners exposed to Libby amphibole. Computed tomography (CT) permits the detection of the characteristic thin, lamellar pleural thickening (LPT).

METHODS

Individuals who worked at the mine for a minimum of 6 months had chest CT and pulmonary function tests.

RESULTS

Pleural thickening was noted in 223 (87%) of the 256 miners, parenchymal abnormalities in 49 (19%). LPT, found in 151 (68%), was associated with low values of forced vital capacity and diffusion capacity and significantly lower values in all pulmonary function tests when associated with parenchymal abnormalities.

CONCLUSION

Eighty-seven percent of miners exposed to Libby Amphibole had pleural abnormalities on CT. LPT alone, and more so with parenchymal abnormalities, resulted in decreased pulmonary function. The importance of this easily missed LPT is demonstrated by its high frequency and significant functional effects.

Quantification of pleural plaques by computed tomography and correlations with pulmonary function: preliminary study

Background

The aims of this study were to quantify pleural plaque volumes (PPVs) by computed tomography (CT) and investigate relations between PPV and pulmonary function after excluding other factors that might affect pulmonary function.

Methods

Twenty-six subjects with pleural plaques and pulmonary function test (PFT) results among 1,544 subjects that registered with the Korea Environment Corporation for asbestos damage relief from January 2011 to December 2015 were included. Subjects with CT evidence of lung diseases and/or previous surgery were excluded. PPVs were measured by tracing the outlines of all pleural plaques on CT images. Patients were allocated to three groups by PPV, as follows, <10, 10-20, or ≥20 mL, and the PFT results of these groups were analyzed and compared. Simple linear regression analysis and multiple regression analysis were used to evaluate correlations between PPV and PFT variables.

Results

No significant relationship was found between total PPV and pulmonary function indices or between PPV groups and PFT results (P>0.05). However, forced expiratory volume in 1 second (FEV₁), forced vital capacity (FVC), and diffusing capacity of the lung for carbon monoxide (DLCO) values in the higher PPV group (≥20 mL) tended to be lower, indicating a restrictive pattern of pulmonary function.

Conclusions

We quantified PPV on CT and found out higher PPV tended to exhibit a restrictive pattern. However, pleural plaques alone were not found to be significantly associated with pulmonary function.

Pleural abnormalities in the Framingham Heart Study: prevalence and CT image features

BACKGROUND

The prevalence of pleural abnormalities in the general population is an epidemiologically important index of asbestos exposure, which has not been investigated since a radiography-based study in 1980.

METHODS

We examined 2633 chest CT scans (mean 59.2 years, 50% female) from the Framingham Heart Study (FHS) for the presence and image characteristics of pleural plaques and diffuse pleural thickening. Demographics and pulmonary function were stratified by the presence of pleural abnormalities in association with interstitial lung abnormalities.

RESULTS

Pleural abnormalities were present in 1.5% (95% CI 1.1% to 2.1%). Pleural lesions were most commonly bilateral (90.0%), multiple (77.5%), calcified (97.5%) and commonly involved posterior (lower: 92.5%, middle: 87.5%), anterior (upper: 77.5%, middle: 77.5%) and diaphragmatic areas (72.5%). Participants with pleural abnormalities were significantly older (75.7 years, p <0.0001), male (92.5%, p <0.0001), former or current smokers (80.0%, p <0.001) with higher pack-years (33.3, p <0.0001). No significant reduction was noted in pulmonary function measures (p=0.07-0.94) when adjusted for the associated covariates, likely due to small number of cases with pleural abnormalities. Information about prior history of asbestos exposure and occupation was not available.

CONCLUSIONS

Pleural plaques and diffuse pleural thickening are present on CT in 1.5% of the FHS cohort. The current prevalence of the pleural abnormalities is smaller than that reported in the previous population-based study using chest radiography, likely representing lower asbestos exposure in recent decades. The posterior portion of the pleura is most frequently involved but the anterior portion is also commonly involved.

The Effects of Pleural Plaques on Longitudinal Lung Function in Vermiculite Miners of Libby, Montana

BACKGROUND

This study was conducted to assess associations of pleural plaques (PP) and longitudinal lung function in vermiculite miners of Libby, Montana who are occupationally exposed to asbestos. High-resolution computed tomography (HRCT) was used to identify asbestos-related findings in former Libby vermiculite miners. We investigated annual lung function decline in miners with PP only and compared them to miners with normal HRCT findings.

MATERIALS AND METHODS

HRCTs from 128 miners were categorized into the following 4 diagnostic groups: (1) normal computed tomography scan (n = 9); (2) PP only (n = 72); (3) PP and interstitial fibrosis (n = 26) and (4) additional HRCT abnormalities (n = 21) such as rounded atelectasis, diffuse pleural thickening, pleural effusions or pulmonary nodules or tumor >1cm in diameter. Random intercept and slope linear mixed-effect regression models identified differences in lung function decline between miners with asbestos-associated outcomes and those with normal HRCT. Models were adjusted for follow-up time, body mass index, smoking status, latent exposure period and employment years. Interactions for smoking status with age and smoking status with pleural plaque severity were examined.

RESULTS

Miners with PP only did not have an accelerated decline in lung function between 40 and 80 years. Miners with PP and additional HRCT abnormalities displayed significantly accelerated declines in forced expiratory volume in 1 second and diffusing capacity of the lungs for carbon monoxide (P = 0.05 and P < 0.01, respectively). Plaque severity did not affect lung function decline. However, smokers with extensive plaques displayed accelerated loss in diffusing capacity of the lungs for carbon monoxide and forced expiratory volume in 1 second when compared to nonsmoking miners with mild plaque formation.

CONCLUSIONS

PP alone did not significantly affect lung function decline in vermiculite miners of Libby, Montana.

Long-term toxicity of naturally occurring asbestos in male Fischer 344 rats

Naturally occurring asbestos (NOA) fibers are found in geologic deposits that may be disturbed by mining, earthworks, or natural processes, resulting in adverse health risks to exposed individuals. The toxicities of Libby amphibole and NOA samples including Sumas Mountain chrysotile (SM), El Dorado tremolite (ED), and Ontario ferroactinolite cleavage fragments (ON) were compared in male Fischer 344 (F344) rats 15 mo after exposure. Rat-respirable fractions of LA and SM displayed greater mean lengths and aspect ratios than ED and ON. After a single intratracheal (IT) instillation (0.5 or 1.5 mg/rat), persistent changes in ventilatory parameters and a significant increase in lung resistance at baseline and after methacholine aerosol dosing were found only in rats exposed to 1.5 mg SM. High-dose ED significantly elevated bronchoalveolar lavage lactate dehydrogenase (LDH) activity and protein levels, while high-dose SM increased γ-glutamyl transferase and LDH activities. A moderate degree of lung interstitial fibrosis after exposure to 1.5 mg SM persisted 15 mo after exposure, unchanged from previous findings at 3 mo. LA induced mild fibrosis, while ED and ON produced minimal and no apparent fibrosis, respectively. Bronchioloalveolar carcinoma was observed 15 mo after exposure to LA or ED. Data demonstrated that SM, given by bolus IT dosing on an equivalent mass basis, induced greater pulmonary function deficits, airway hyperresponsiveness, and interstitial fibrosis than other NOA, although unlike LA and ED, no apparent evidence for carcinogenicity was found. All NOA samples except ON cleavage fragments produced some degree of long-term toxicity.

Exposure-response modeling of non-cancer effects in humans exposed to Libby Amphibole Asbestos; update

The United States Environmental Protection Agency (EPA) developed a quantitative exposure-response model for the non-cancer effects of Libby Amphibole Asbestos (LAA) (EPA, 2014). The model is based on the prevalence of localized pleural thickening (LPT) in workers exposed to LAA at a workplace in Marysville, Ohio (Lockey et al., 1984; Rohs et al., 2008). Recently, Lockey et al. (2015a) published a follow-up study of surviving Marysville workers. The data from this study increases the number of cases of LPT and extends the observation period for a number of workers, thereby providing a strengthened data set to define and constrain the optimal exposure-response model for non-cancer effects from inhalation exposure to LAA. The new data were combined with the previous data to update the exposure-response modeling for LPT. The results indicate that a bivariate model using cumulative exposure and time since first exposure is appropriate, and the benchmark concentration is similar to the findings previously reported by EPA (2014). In addition, the data were also used to develop initial exposure-response models for diffuse pleural thickening (DPT) and small interstitial opacities (SIO).

Systematic review of pleural plaques and lung function

Context

US EPA proposed a Reference Concentration for Libby amphibole asbestos based on the premise that pleural plaques are adverse and cause lung function deficits.

Objective

We conducted a systematic review to evaluate whether there is an association between pleural plaques and lung function and ascertain whether results were dependent on the method used to identify plaques.

Methods

Using the PubMed database, we identified studies that evaluated pleural plaques and lung function. We assessed each study for quality, then integrated evidence and assessed associations based on the Bradford Hill guidelines. We also compared the results of HRCT studies to those of X-ray studies.

Results

We identified 16 HRCT and 36 X-ray studies. We rated six HRCT and 16 X-ray studies as higher quality based on a risk-of-bias analysis. Half of the higher quality studies reported small but statistically significant mean lung function decrements associated with plaques. None of the differences were clinically significant. Many studies had limitations, such as inappropriate controls and/or insufficient adjustment for confounders. There was little consistency in the direction of effect for the most commonly reported measurements. X-ray results were more variable than HRCT results. Pleural plaques were not associated with changes in lung function over time in longitudinal studies.

Conclusion

The weight of evidence indicates that pleural plaques do not impact lung function. Observed associations are most likely due to unidentified abnormalities or other factors.