A 65-year-old female patient sought medical attention for abdominal distention and shortness of breath that persisted for 15 d. The aforementioned symptoms appeared half a month ago with no apparent cause, and the patient felt as if her symptoms were worsening. She had a history of high blood pressure and type 2 diabetes but no history of asbestos exposure. The patient denied any family history of malignant tumors. On admission to the hospital, she had a distended abdomen, with an abdominal circumference of 111 cm, slight tension in the abdominal muscles, mild tenderness, and mobile dullness. She reported a weight gain of 5 kg in 1 mo. The findings of blood tests and routine examination of ascites are shown in Table. After collection, ascitic fluid was sent for pathological examination and immunohistochemistry. The final report of liquid-based cell preparation and cell-block hematoxylin and eosin (HE)-stained section indicated numerous atypical cells, some of which had papillary structures. The immunohistochemical (IHC) markers were as follows: CK (+), CK20 (-), Villin (-), CDX2 (-), CR (+), WT-1 (-), SATB-2 (scattered +), Ki-67 (about 10% +), TTF-1 (-), CEA (-), Pax-8 (-), P16 (individual +), M-mell (-), and Glut-1 (+++), and desmin (-). fluorescence in situ hybridization (FISH) detected CDKN2A (P16) gene deletion and PD-L1 (-). The contrast-enhanced computed tomography (CT) showed a large volume of ascites in the abdominal cavity, but no abdominal wall thickening or intra-abdominal soft-tissue shadow was observed. However, subsequent pelvic enhancement magnetic resonance imaging (MRI) revealed mild peritoneal thickening.