An 83-year-old man was admitted to our hospital complaining of left lower abdominal pain for a month. Abdominal ultrasonography and computed tomography showed descending and sigmoid colon wall thickening, left hydronephrosis and a small amount of ascites. Colonoscopy showed edematous mucosa and stenosis of sigmoid colon, so the scope could not pass the lesion. Histological examination of biopsy specimens from the colonoscopy showed non-neoplastic cells. Malignant cells were detected by cytological examination of ascitic fluid. The immunostaining pattern (CK7-, CK20+, CDX2+) suggested colorectal cancer. After palliative care, the patient died. A pathological autopsy showed type 4 colorectal cancer with peritoneal dissemination. Type 4 colorectal cancer is rare and accounts for only 0.5-1.3% of all colorectal cancer. Diagnosis of type 4 colorectal cancer is difficult because the detection rate by biopsy is low. The cytodiagnosis of ascites combined with immunostaining is useful for diagnosis of type4 colorectal cancer with malignant ascites.
