Peritoneal Fluid: HGSC Case

A pathology case shared online this month walked through how cancer cells in abdominal fluid can point to high-grade serous carcinoma, the most common lethal form of ovarian-type cancer. (x.com) Peritoneal fluid is the liquid in the abdominal cavity, and when it builds up as ascites, doctors can examine the cells floating in it under a microscope. In cancer care, that fluid can contain shed tumor cells from ovarian, fallopian tube, or primary peritoneal cancers. (cancer.org) High-grade serous carcinoma is usually diagnosed by combining cell shape with immunohistochemistry, a stain-based test that works like a molecular barcode on a cell block. A large cytology series found ascitic fluid from these tumors often shows moderate to high cellularity with papillary clusters, while tumor cells commonly stain for cytokeratin 7, paired box gene 8, and Wilms tumor 1. (academic.oup.com) In the shared case, the reported stain pattern was cytokeratin 7 positive, paired box gene 8 positive, Wilms tumor 1 positive, p16 block positive, and p53 null. That combination fits the standard profile of high-grade serous carcinoma, which typically shows paired box gene 8 and Wilms tumor 1 positivity plus an abnormal p53 pattern and strong diffuse p16 expression. (x.com) (pathologyoutlines.com) The p53 result matters because a “null” pattern means tumor-cell nuclei fail to stain, one of the abnormal patterns that usually reflects a TP53 gene alteration. In one study of 51 cytology specimens with p53 testing, 17 cases, or 33%, showed this null-type pattern, and the p53 result matched later tissue diagnosis in 100% of cases. (pubmed.ncbi.nlm.nih.gov) The differential diagnosis is the hard part in fluid cytology because several cancers can look crowded and atypical in a smear. Mesothelioma can overlap with serous carcinoma, but paired box gene 8 generally favors Müllerian carcinoma over mesothelial disease, while calretinin and cytokeratin 5 or 6 support mesothelioma. (pmc.ncbi.nlm.nih.gov) (pathologyoutlines.com) The same panel also helps separate this tumor from colorectal metastasis and endometrioid carcinoma. High-grade serous carcinoma is usually cytokeratin 7 positive and Wilms tumor 1 positive, while colorectal adenocarcinoma more often follows a cytokeratin 7 negative, cytokeratin 20 positive, caudal type homeobox 2 positive pattern, and endometrioid tumors are less often Wilms tumor 1 positive. (pmc.ncbi.nlm.nih.gov 1) (pmc.ncbi.nlm.nih.gov 2) That distinction affects treatment because ovarian epithelial, fallopian tube, and primary peritoneal cancers are staged and treated together, often with surgery, platinum-based chemotherapy, targeted therapy, or poly (ADP-ribose) polymerase inhibitor maintenance in selected patients. The National Cancer Institute and National Comprehensive Cancer Network both group these diseases in the same treatment framework. (cancer.gov) (nccn.org) The biology behind the diagnosis has shifted over the past decade. Research now supports that many tumors long labeled “ovarian” high-grade serous carcinoma actually begin in the fallopian tube, then spread across the peritoneal surfaces where malignant fluid can accumulate. (nature.com) (cancer.gov) The case resonated because it showed the full chain of evidence in one specimen: abnormal cells in fluid, a short stain panel, and a differential diagnosis narrowed in real time. In peritoneal fluid cytology, that is often the difference between calling a generic metastatic carcinoma and naming high-grade serous carcinoma with confidence. (x.com) (academic.oup.com)