Abstract Introduction Isolated pleural presentation of lung adenocarcinoma, in the absence of identifiable primary lung tumor elsewhere in the parenchyma, is exceedingly rare and has been described almost exclusively in single case reports rather than large series. We present an unusual case of pleural-limited adenocarcinoma mimicking mesothelioma radiographically. Case Description A 67-year-old woman with an extensive history of tobacco use presented with several months of progressive exertional dyspnea and unintentional weight loss. Computed tomography (CT) of the chest revealed a large right pleural effusion with circumferential pleural thickening and mediastinal/hilar lymphadenopathy. A pigtail catheter was placed for drainage. Pleural fluid analysis showed lymphocyte-predominant, exudative effusion containing rare atypical and mesothelial cells. Repeat CT chest confirmed persistent nodular pleural thickening with no primary lung tumor seen in the parenchyma Figure 1. Cytologic evaluation of the pleural fluid demonstrated atypical epithelial cells positive for MOC-31 and TTF-1 and negative for calretinin and WT-1; molecular testing was positive for BRAF, TP53, and MYC, consistent with metastatic lung adenocarcinoma. Figure 1. CT chest before (A) and after (B) pleural fluid drainage revealing nodular pleural thickening. Discussion While pleural metastases are common in advanced lung cancer, with malignant pleural effusion seen in up to 15% of non-small cell lung carcinoma cases 1, presentation limited solely to the pleura without a lung nodule is uncommon 2. Only a few case reports of localized pleural adenocarcinoma have been published, underscoring its rarity. Radiographically, such cases may mimic primary pleural malignancies, including mesothelioma, or other entities such as pleural lymphoma or metastases from extrathoracic primaries, particularly when imaging demonstrates diffuse pleural thickening without a parenchymal lesion. Pleural thickening extending into the mediastinum is classically associated with mesothelioma, further obscuring diagnosis and contributing to misclassification 3. Immunohistochemistry remains essential: mesothelioma expresses calretinin, WT-1, and D2-40, whereas adenocarcinoma expresses MOC-31, Ber-EP4, TTF-1, and Napsin A. Recognizing pleural-limited lung adenocarcinoma is critical, as it carries distinct therapeutic and prognostic implications. Accurate diagnosis ensures timely initiation of targeted or systemic therapy and appropriate oncologic management. References: 1. Divisi D, Di Leonardo G, Crisci R. Malignant pleural effusion in lung cancer: focus on treatment. J Xiangya Med. 2020;5:42. doi:10.21037/jxym-20-42 / 2. Agalioti T, Giannou AD, Stathopoulos GT. Pleural involvement in lung cancer. J Thorac Dis. 2015;7(6):1021-1030. doi:10.3978/j.issn.2072-1439.2015.03.19 / 3. Metintas M, Ucgun I, Elbek O, et al. Computed tomography features in malignant pleural mesothelioma and other commonly seen pleural diseases. Eur J Radiol. 2002;41(1):1-9. doi:10.1016/S0720-048X(01)00426-0 This abstract is funded by: None
Rahman et al. (Fri,) studied this question.