A 38-year-old woman from Mumbai presents with a 6-month history of progressive abdominal distension. She denies vaginal bleeding or constitutional symptoms. On examination, there is marked ascites and a large, smooth, firm abdominal mass. Serum CA-125 is 320 U/mL. Ultrasound reveals a large, predominantly cystic ovarian mass with thick septa and locules, and ascites. Histopathology shows columnar epithelium with mucin production, focal stratification, and mild nuclear atypia. What is the most likely diagnosis?

Explanation

## Clinical and Pathologic Integration **Key Point:** A woman of reproductive age presenting with a large, multiloculated cystic ovarian mass, ascites, elevated CA-125, and histology showing columnar mucinous epithelium with mild atypia and stratification is most consistent with mucinous cystadenocarcinoma (MCC), a low-grade epithelial ovarian malignancy. ## Mucinous Tumors of the Ovary — Spectrum | Feature | Cystadenoma | Borderline Tumor | Cystadenocarcinoma | |---------|-------------|------------------|--------------------| | **Age** | 20–50 yr | 30–50 yr | 30–50 yr | | **Size** | Variable, often large | Large (>10 cm) | Large (>10 cm) | | **CA-125** | Normal or <100 U/mL | Mildly elevated (100–300 U/mL) | Elevated (>200 U/mL) | | **Ascites** | Absent (unless rupture) | Absent or minimal | Present (pseudomyxoma peritonei if ruptured) | | **Epithelial Atypia** | Absent | Mild to moderate | Moderate to marked | | **Invasion** | None | Absent | Present (into stroma) | | **Prognosis** | Excellent (>99%) | Intermediate (~90% 5-yr OS) | Poor if ruptured (~50% 5-yr OS); better if confined (~80%) | ## Histopathologic Criteria for Mucinous Cystadenocarcinoma 1. **Epithelial lining:** Tall columnar mucinous cells (resembling intestinal epithelium) 2. **Mucin production:** Abundant intracellular and extracellular mucin (PAS+, mucicarmine+) 3. **Nuclear atypia:** Mild to moderate; nuclei enlarged, hyperchromatic, irregular membranes 4. **Stratification:** Focal to multifocal epithelial stratification (>1 layer) 5. **Stromal invasion:** Infiltration of tumor cells into ovarian stroma (key differentiator from borderline tumors) 6. **Mitotic activity:** Increased but not excessive **High-Yield:** Mucinous cystadenocarcinoma is a **low-grade malignancy**. When confined to the ovary (stage I), it has a favorable prognosis (~80–90% 5-yr OS). However, if ruptured, mucinous material spills into the peritoneum, causing pseudomyxoma peritonei (PMP) with worse outcomes. ## Why This Patient Has MCC (Not Cystadenoma or Borderline) - **Ascites + elevated CA-125 (320 U/mL)** suggests malignancy; benign cystadenoma rarely causes ascites and CA-125 is typically <100 U/mL - **Histology shows mild atypia + stratification + stromal invasion** = carcinoma (not benign cystadenoma, which has no atypia or invasion) - **Age 38 yr, reproductive age** = typical for mucinous tumors (not serous, which peaks postmenopausally) - **Large multiloculated cystic mass** = classic imaging for mucinous tumor **Clinical Pearl:** Mucinous tumors account for only ~5–10% of epithelial ovarian cancers but are the most common epithelial tumor in younger women. Always consider mucinous cystadenocarcinoma in a woman <50 yr with a large cystic ovarian mass and ascites. ## Distinction from Borderline Mucinous Tumor Borderline mucinous tumors lack stromal invasion and have better prognosis (~95% 5-yr OS if stage I). The presence of stromal invasion in this case elevates the diagnosis to cystadenocarcinoma. ## Management **Tip:** Stage I mucinous cystadenocarcinoma is treated with surgery alone (unilateral salpingo-oophorectomy ± staging). Chemotherapy is reserved for advanced or ruptured tumors. Unlike serous carcinoma, mucinous tumors are relatively chemoresistant.