A 52-year-old postmenopausal woman presents with progressive abdominal distension and mild abdominal pain for 3 months. On examination, she has ascites and a palpable abdominal mass. Serum CA-125 is markedly elevated at 680 U/mL. Imaging shows a large complex ovarian mass with internal septations and solid components. Ascitic fluid cytology reveals malignant epithelial cells. What is the most likely diagnosis?

## Clinical Diagnosis: Serous Cystadenocarcinoma **Key Point:** Serous cystadenocarcinoma is the most common malignant epithelial ovarian tumor, accounting for ~40% of epithelial ovarian cancers. It typically presents in postmenopausal women with advanced-stage disease. ### Clinical Features Supporting This Diagnosis | Feature | Serous Cystadenocarcinoma | Mucinous Cystadenoma | Granulosa Cell Tumor | Dysgerminoma | | --- | --- | --- | --- | --- | | **Age** | Postmenopausal (50–70 yrs) | Postmenopausal | Perimenopausal | Young women (10–30 yrs) | | **Presentation** | Advanced disease, ascites | Usually early stage | Hormone symptoms | Young, often early stage | | **CA-125** | Markedly elevated | Normal/low | Normal | Normal | | **Ascites** | Common in advanced stage | Rare | Rare | Rare | | **Malignant Potential** | High (40% of epithelial cancers) | Low (5% malignant) | Low-grade | Germ cell origin | **High-Yield:** Serous tumors account for ~50% of all epithelial ovarian tumors (benign + malignant combined). When malignant, they present with: - Ascites and peritoneal seeding - Markedly elevated CA-125 (>500 U/mL common) - Complex, solid-cystic masses with internal septations - Malignant cells on ascitic fluid cytology ### Pathologic Features 1. **Gross appearance:** Large, complex cystic mass with solid areas, papillary projections 2. **Microscopy:** Cuboidal to columnar epithelium with papillary architecture; high mitotic rate; nuclear atypia 3. **Immunohistochemistry:** WT-1 positive, PAX8 positive (helps confirm Müllerian origin) **Clinical Pearl:** Serous cystadenocarcinomas are often diagnosed at FIGO stage III–IV because they seed the peritoneum early. The presence of ascites with malignant cells on cytology indicates peritoneal involvement (stage III minimum). **Mnemonic — Serous Cystadenocarcinoma RED FLAGS:** **ASCITES** = Advanced stage, Serum CA-125 elevated, Cytology malignant, Imaging complex, Typically postmenopausal, Epithelial origin, Solid papillary architecture. ### Why This Diagnosis Fits - Postmenopausal age - Markedly elevated CA-125 (680 U/mL) - Ascites with malignant cytology (peritoneal seeding) - Complex ovarian mass with solid components - Progressive abdominal distension (classic presentation of advanced epithelial ovarian cancer)