Diffuse intrapulmonary malignant mesothelioma presenting as spontaneous hydropneumothorax and non-resolving pneumonia: Diagnosis by transbronchial biopsy

ABSTRACT Diffuse intrapulmonary malignant mesothelioma (DIMM) is a rare mesothelioma variant characterized by predominant lung parenchymal involvement with minimal or inconspicuous pleural disease. It may present as persistent pulmonary consolidation initially attributed to pneumonia and can mimic interstitial lung disease or lepidic adenocarcinoma. A 77-year-old lifelong non-smoker without occupational asbestos exposure presented with spontaneous hydropneumothorax. Computed tomography revealed a large right pleural effusion and moderate right pneumothorax with an air-fluid level, without pleural thickening, enhancement, or a discrete mass. Despite negative pleural cytology and biopsy findings, he developed persistent right lower lobe consolidation and later re-presented with progressive respiratory and constitutional symptoms. Bronchoscopy with transbronchial biopsies revealed epithelioid mesothelioma with a mesothelial immunophenotype and BRCA1-associated protein 1 loss, consistent with DIMM. This case highlights DIMM as a rare cause of persistent unilateral pulmonary opacities initially attributed to pneumonia. Such abnormalities after spontaneous hydropneumothorax should raise suspicion for malignancy despite initially negative pleural investigations.