Non-Small Cell Lung Carcinoma Presenting as Isolated Malignant Ascites in the Absence of a Detectable Primary Tumor: A Case Report and Literature Review.

An 89-year-old woman presented with abdominal distension and altered mental status. Non-portal-hypertensive ascites suggested spontaneous bacterial peritonitis, but ascitic-fluid cytology with immunoprofiling and circulating tumor-DNA confirmed non-small cell lung carcinoma despite no detectable primary tumor, underscoring early consideration of malignancy in unexplained ascites.