Large cell neuroendocrine carcinoma of unknown primary presenting with pleural fluid: A diagnostic pitfall with lymphoma-like features.

Large-cell neuroendocrine carcinoma (LCNEC), a rare and aggressive malignancy, most commonly arising in the lungs but infrequently presenting at unusual locations or with atypical features. Diagnosing LCNEC can be particularly challenging when the primary site is unknown and when clinical and radiologic findings mimic other malignancies. Here, we describe a case involving an elderly male who presented only with generalized lymphadenopathy and pleural effusion, initially raising suspicion for a hematologic neoplasm. Detailed cytological and immunocytochemical evaluation of pleural fluid played a pivotal role in reaching the final diagnosis. This case underscores the diagnostic complexity of LCNEC with unknown primary and highlights the importance of cytological analysis in cases where conventional diagnostic approaches are limited.