Malignant mesothelioma typically encases lungs as a thick rind, while relatively sparing lung parenchyma. We describe an unusual presentation of mesothelioma characterized by diffuse intrapulmonary growth, with absent or inconspicuous pleural involvement, clinically simulating interstitial lung disease (ILD). We identified 5 patients (median age 56 y, all men) with diffuse intrapulmonary malignant mesothelioma in our pathology consultation practice from 2009 to 2012. Clinical history, imaging, and pathology materials were reviewed. Symptoms included chronic dyspnea (4 cases), cough (3), and acute dyspnea with bilateral pneumothorax (1). Chest imaging showed irregular opacities (5), reticulation (4), pleural effusions (2), and subpleural nodular densities (1), without radiologic evidence of pleural disease or masses. A clinicoradiologic diagnosis of ILD was made in all cases, and wedge biopsies were performed. Histologic evaluation revealed a neoplastic proliferation of bland epithelioid or spindled cells, showing various growth patterns simulating silicotic nodules, desquamative interstitial pneumonia, organizing pneumonia, and Langerhans cell histiocytosis. Some areas mimicked adenocarcinoma, with lepidic, acinar, micropapillary, and solid patterns. Initial diagnoses by referring pathologists included reactive changes (1), hypersensitivity pneumonitis versus drug reaction (1), desquamative interstitial pneumonia versus neoplasm (1), and mesothelioma (2). Microscopic pleural involvement was identified in 4 cases. Immunohistochemistry confirmed the characteristic immunophenotype of mesothelioma in all cases. Median survival of 3 patients treated with chemotherapy was 28 months. Two patients received no therapy and survived 3 and 4 weeks, respectively. "Diffuse intrapulmonary malignant mesothelioma" is a rare variant with a distinctive presentation that clinically mimics ILD. Recognition is essential to avoid misdiagnosis.

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