Historical shift in pathological type of progressive massive fibrosis among coal miners in the USA

Abstract

Background: Pneumoconiosis among coal miners in the USA has been resurgent over the past two decades, despite modern dust controls and regulatory standards. Previously published studies have suggested that respirable crystalline silica (RCS) is a contributor to this disease resurgence. However, evidence has been primarily indirect, in the form of radiographic features.

Methods: We obtained lung tissue specimens and data from the National Coal Workers' Autopsy Study. We evaluated specimens for the presence of progressive massive fibrosis (PMF) and used histopathological classifications to type these specimens into coal-type, mixed-type and silica-type PMF. Rates of each were compared by birth cohort. Logistic regression was used to assess demographic and mining characteristics associated with silica-type PMF.

Results: Of 322 cases found to have PMF, study pathologists characterised 138 (43%) as coal-type, 129 (40%) as mixed-type and 55 (17%) as silica-type PMF. Among earlier birth cohorts, coal-type and mixed-type PMF were more common than silica-type PMF, but their rates declined in later birth cohorts. In contrast, the rate of silica-type PMF did not decline in cases from more recent birth cohorts. More recent year of birth was significantly associated with silica-type PMF.

Conclusions: Our findings demonstrate a shift in PMF types among US coal miners, from a predominance of coal- and mixed-type PMF to a more commonly encountered silica-type PMF. These results are further evidence of the prominent role of RCS in the pathogenesis of pneumoconiosis among contemporary US coal miners.

Keywords: Coal Mining; Dust; Lung Diseases, Interstitial; Respiratory System; Silicosis.

Figure 1

Representative examples of silica, coal and mixed types of PMF, (H&E stains). (A) Silica-type PMF (>75% silicotic nodules). This lesion of silica-type PMF, extending from the upper portion of the section to bottom-right quadrant, consists of a conglomeration of rounded silicotic nodules (solid black circles) admixed with a moderate degree of black coal pigment. Black coal pigment is present, but to a lesser extent than is visible in the coal or mixed-type PMF lesions. This silica-type lesion is transected at top and bottom of the section, indicating that the actual lesion is larger than is visible on this slide. Also evident are silicotic nodules (≤10 mm in greatest linear dimension) in the surrounding lung tissue (dotted black circle). The remainder of the lung parenchyma shows scattered coal macules. V=vessel. (B) Coal-type PMF lesion (≤25% silicotic nodules). In contrast to the silica-type PMF, this lesion consists of dense coal deposits associated with underlying fibrosis resulting in a large lesion measuring greater than 10 mm in greatest linear dimension. There is focal necrosis with cavitation typified by the cystic appearing space partially filled with degenerating material (blue star). No significant silicotic nodules are identified within this lesion. The surrounding lung parenchyma shows abundant coal macules (example in black circle) and nodules associated with centriacinar emphysema. V=vessel, B=bronchiole. (C) Mixed-type PMF (>25% and ≤75% silicotic nodules). The mixed type of PMF shows both silicotic nodules (example in black circle) and dense coal deposits resulting in a lesion greater than 10 mm in greatest linear dimension. The silicotic nodule component consists of rounded to elongated parallel collagen fibres. This lesion was partially transected at bottom of the section, indicating that this lesion is larger than is visible on this single slide. To the right of the PMF lesion, there is a portion of a coal nodule (blue arrow). The remainder of the lung parenchyma shows emphysematous changes (E=area of emphysema). PMF, progressive massive fibrosis.

Figure 2

Rates of PMF and PMF types relative to NCWAS population by 5-year birth cohort. Best-fit trend lines for PMF types do not include periods in which no cases of that type were identified (eg, coal-type PMF for 1940–1944 birth cohort). NCWAS, National Coal Workers’ Autopsy Study; PMF, progressive massive fibrosis.