A 68-year-old woman undergoes imaging for chronic pelvic pain and is found to have a large, multiloculated cystic ovarian mass measuring 18 cm. The mass is predominantly cystic with minimal solid component. Serum CA-125 is 45 U/mL (normal <35 U/mL, only mildly elevated). There is no ascites. Histopathology shows tall columnar mucinous epithelium lining the cyst, with areas of mucin extravasation into the stroma but no invasion of the ovarian capsule. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Mucinous Cystadenoma **Key Point:** The defining feature that separates **mucinous cystadenoma** (benign) from **mucinous cystadenocarcinoma** (malignant) is the presence or absence of **true stromal invasion**. Mucin extravasation into the stroma alone — without frank stromal invasion or capsular breach — does NOT constitute malignancy. This case explicitly states **no invasion of the ovarian capsule**, making mucinous cystadenoma the correct diagnosis. ### Diagnostic Criteria: Mucinous Cystadenoma vs. Cystadenocarcinoma | Feature | Mucinous Cystadenoma | Mucinous Cystadenocarcinoma | | --- | --- | --- | | **Stromal invasion** | Absent | Present (frank infiltrative or expansile invasion) | | **Capsule** | Intact | Breached or ruptured | | **Epithelial atypia** | Minimal or absent | Moderate to severe | | **Size** | Often large (10–30 cm) | Often large; size alone ≠ malignancy | | **CA-125** | Normal or mildly elevated | Mildly to moderately elevated | | **Ascites** | Absent | May be present | | **Prognosis** | Excellent (>95% 5-yr survival) | ~50–60% 5-yr survival | **High-Yield (Robbins Pathology):** Mucinous cystadenomas are the **most common benign ovarian epithelial tumors** and can grow to enormous sizes (10–30 cm). They are lined by tall columnar mucinous epithelium resembling endocervical or intestinal epithelium. Mucin extravasation into the stroma can occur as a secondary phenomenon (e.g., cyst rupture) and does **not** by itself indicate malignancy. True carcinoma requires **demonstrable stromal invasion** (irregular glands infiltrating stroma with desmoplastic reaction) or **capsular breach**. ### Pathologic Interpretation of This Case 1. **Tall columnar mucinous epithelium** — hallmark of mucinous differentiation ✓ 2. **Mucin extravasation into stroma** — a secondary finding, NOT equivalent to stromal invasion ✓ 3. **No invasion of the ovarian capsule** — explicitly stated; this rules out carcinoma ✓ 4. **Predominantly cystic, minimal solid component** — favors benign/borderline ✓ 5. **Only mildly elevated CA-125 (45 U/mL)** — consistent with benign mucinous tumor ✓ 6. **No ascites** — further supports benign process ✓ This combination = **Mucinous cystadenoma**, not cystadenocarcinoma. ### Why the Other Options Are Incorrect - **A) Clear cell carcinoma:** Characterized by hobnail cells, clear cytoplasm, and solid/papillary architecture — not mucinous epithelium. - **B) Serous cystadenocarcinoma:** Shows psammoma bodies, papillary architecture, and serous (not mucinous) epithelium; typically associated with markedly elevated CA-125. - **C) Mucinous cystadenocarcinoma:** Requires frank stromal invasion or capsular breach — neither is present in this case. Mucin extravasation alone is insufficient for this diagnosis. **Clinical Pearl:** A **mucinous borderline tumor (atypical proliferative mucinous tumor)** would be considered if there is epithelial stratification and mild atypia without stromal invasion — but the stem describes no atypia and no invasion, making straightforward cystadenoma the best fit. Per Robbins (10th ed.), the diagnosis of carcinoma requires unequivocal stromal invasion. **Mnemonic — Mucinous Cystadenoma:** **No Invasion = No Carcinoma**. Large size + mucinous epithelium + intact capsule + no stromal invasion = Benign cystadenoma.