Study Syllabus for Classification of Radiographs of Pneumoconioses
Pathology Basis of Occupational Lung Disease
Asbestos exposure increases the risk for lung carcinoma of all histological types [Churg 1998b]. The gross and microscopic appearance of lung carcinomas caused by asbestos is identical to that causally associated with tobacco smoke. An important variant of adenocarcinoma to recognize in the setting of asbestos exposure is pseudomesotheliomatous adenocarcinoma, in which a peripheral lung adenocarcinoma extensively infiltrates the pleura, causing pleural thickening that simulates diffuse pleural fibrosis or malignant mesothelioma [Harwood et al. 1976]. On pathological examination the primary adenocarcinoma may be occult and difficult to identify. Other histological types of primary lung cancer or even metastatic tumors involving the pleura may generate a pseudomesotheliomatous appearance. In the series of pseudomesotheliomatous adenocarcinomas of Koss et al, 17% had possible or definite asbestos exposure [Koss et al. 1992].
Asbestos exposure is the most important cause of pleural mesothelioma, a tumor that originates from the serosal surface [Hammar et al. 2008]. The classical macroscopic appearance of malignant mesothelioma is that of a diffuse, thick rind of tumor encasing the lung, extending along the interlobar fissures, and infiltrating the chest wall, diaphragm, and mediastinal structures (Fig. 40). The pleural rind is usually thickest in the dependent regions of the thorax. Pleural mesotheliomas may exhibit nodularity and/or loculations filled with hemorrhagic fluid. The ipsilateral lung parenchyma is typically compressed by the expanding tumor burden.
Mesotheliomas are histologically diverse tumors with many different patterns and subtypes [Hammar et al. 2008]. The majority of mesotheliomas, however, fall into one of the categories of epithelioid, sarcomatoid, or biphasic subtypes, the latter having features of both sarcomatous and epithelial morphology. While mesotheliomas in general are regarded as nearly uniformly fatal, the sarcomatoid subtype carries an especially poor prognosis. Among sarcomatoid mesotheliomas, the desmoplastic variant often causes diagnostic challenges in the histological distinction from diffuse pleural fibrosis. In general, the histological separation of epithelioid mesothelioma from metastatic adenocarcinoma requires the use of a panel of immuno-histochemical stains, including relatively mesothelioma-specific markers like calretinin, cytokeratin 5/6, and WT-1, in addition to adenocarcinoma markers such as carcinoembryonic antigen, thyroid transcription factor 1, CD15, and B72.3. The distinction of sarcomatoid mesothelioma from primary or metastatic sarcomas of the pleura may be facilitated by immunostaining for keratin, which is usually positive in sarcomatoid mesotheliomas [Hammar et al. 2008].