Large Pleural Effusion Secondary to Primary Signet-Ring Cell Adenocarcinoma of the Lung

Primary signet-ring cell adenocarcinomas (SRCA) that occur outside the gastrointestinal tract are rare with few cases reported in the literature. They carry a poor prognosis and can be a challenge to diagnose. In this case report, we present the case of a 69-year-old female who presented to our hospital with a large left-sided pleural effusion. The findings seen on histopathology and immunohistochemistry (IHC) staining of the pleural fluid were consistent with primary signet-ring cell adenocarcinoma of the lung, and further investigations excluded a secondary source of malignancy. To the best of our knowledge, this is the first reported case of pleural effusion without an underlying lung mass diagnosed as primary signet-ring cell adenocarcinoma of the lung. Given how rare primary signet-ring cell adenocarcinoma of the lung is, it is important for physicians to carry out a comprehensive diagnostic workup to differentiate between primary and metastatic signet-ring cell adenocarcinoma as this will determine prognosis and treatment.