Pseudocarcinomatous Mesothelioma: A Hitherto Unreported Presentation closely simulating primary lung cancer.

Malignant mesothelioma (MM) has a wide range of clinical, radiologic, and pathologic presentations, mimicking lung cancer or interstitial lung diseases when predominantly involving the lung parenchyma. The case herein refers to a 79-year-old man, active smoker without asbestos exposure, incidentally discovered to have a pulmonary nodule in the right upper lobe (1.5 cm). The lesion was misinterpreted as primary lung adenocarcinoma at the frozen section in light of the predominant lepidic growth pattern. Definitive examination confirmed neoplastic proliferation along alveolar structures. However, the unusual globous shape of tumor cells along the alveoli abruptly merging with normal pneumocytes prompted us to perform some immunostains that surprisingly revealed a mesothelial differentiation (positive staining with calretinin, cytokeratins (CK5/6), D2-40, and negativity with BRCA-associated protein 1 (BAP1), Thyroid Transcription Factor 1 [TTF-1], claudin-4, carcinoembryonic antigen [CEA], and napsin). MM represents the pathologic counterpart of so-called pseudomesotheliomatous carcinoma, since it appears as a localized pulmonary neoplastic nodule displaying a predominant lepidic growth pattern (pseudocarcinomatous mesothelioma). The challenging diagnostic features of this unique case and a review of similar cases in the literature are discussed.