Supplementary Material for: Signet Ring Cell Carcinoma of the Stomach with Chylous Pleural Effusion: Laterality Shift and Fluid Composition Change

Introduction: Chylothorax is a rare manifestation of malignant pleural effusion, with gastric cancer accounting for only a small percentage of cases. Signet ring cell carcinoma (SRCC) is an aggressive tumor subtype that frequently invades the lymphatic system, leading to early dissemination.
Case Presentation: A 66-year-old woman presented with bilateral chylothorax that was initially suspected to be a lymphoma. After a transient resolution, the pleural effusion returned on the left side, with a shift from chylous to non-chylous fluid and an increase in carcinoembryonic antigen levels. Despite endoscopic findings resembling early-stage gastric cancer, cytology confirmed SRCC. Despite treatment with capecitabine plus oxaliplatin and nivolumab, the disease progressed, resulting in bilateral effusion recurrence and death within 4 months. Genetic analysis revealed CLDN18.2 positivity.
Conclusion: This case is unique because it demonstrates a rare progression from bilateral to unilateral pleural effusion, accompanied by a transition in pleural fluid characteristics, which has not been previously reported. The observations suggest dynamic changes in lymphatic obstruction and tumor infiltration. Early molecular profiling, which includes CLDN18.2 testing, may provide new opportunities for targeted therapy in aggressive gastric cancer cases.