A 42-year-old multiparous woman presents with a 2-year history of progressive abdominal bloating and mild discomfort. She denies vaginal bleeding or weight loss. Physical examination reveals a palpable suprapubic mass. Transvaginal ultrasound demonstrates a 15 cm unilocular cyst with thin walls, clear fluid, and no septations or solid components. Serum CA-125 is 28 U/mL (normal <35), and CEA is normal. What is the most likely diagnosis?

## Clinical Diagnosis: Serous Cystadenoma ### Key Clinical Features **Key Point:** Serous cystadenoma is the most common benign epithelial ovarian tumor, accounting for 30–40% of benign ovarian neoplasms. It typically presents as a simple, thin-walled unilocular cyst in perimenopausal women. ### Diagnostic Criteria for Serous Cystadenoma | Feature | Serous Cystadenoma | Mucinous Cystadenoma | Serous Cystadenocarcinoma | | --- | --- | --- | --- | | **Age** | 30–50 years (perimenopausal) | 40–60 years | 50–70 years | | **Size** | Usually <10 cm (up to 15 cm) | Often >10 cm, can be massive | Variable | | **Imaging** | Simple, unilocular, thin-walled | Multiloculated, gelatinous, septated | Complex, solid components, ascites | | **Internal fluid** | Clear serous fluid | Gelatinous, mucinous | Hemorrhagic or mixed | | **Tumor markers** | Normal or mildly elevated CA-125 | Elevated CA 19-9, CEA | Elevated CA-125 (often >100) | | **Malignant potential** | 5–10% | 5–10% | Already malignant | | **Rupture risk** | Low | Moderate (pseudomyxoma peritonei if ruptured) | High | ### Why This Case Fits Serous Cystadenoma 1. **Age:** 42 years — typical perimenopausal presentation 2. **Imaging:** Unilocular, thin-walled, clear fluid, no septations or solid components — **pathognomonic for serous cystadenoma** 3. **Size:** 15 cm — within range for benign serous tumors 4. **Tumor markers:** CA-125 normal (28 U/mL), CEA normal — excludes mucinous malignancy and serous carcinoma 5. **Clinical course:** Slow, progressive, asymptomatic except for mass effect **Clinical Pearl:** The key distinguishing feature is the **unilocular, simple cyst with clear fluid**. Mucinous cystadenomas are multiloculated with gelatinous (not clear) fluid. Dermoid cysts show complex internal echoes (fat, hair, teeth). **High-Yield:** Serous cystadenomas are lined by ciliated, columnar epithelium resembling fallopian tube mucosa. They do not produce mucin (unlike mucinous tumors), so CA 19-9 and CEA remain normal. ### Management **Key Point:** Observation with ultrasound surveillance every 6–12 months is appropriate for asymptomatic simple cysts <10 cm. For larger or symptomatic cysts (>10 cm), ovarian-sparing cystectomy via laparoscopy is the treatment of choice in reproductive-age women; hysterectomy with bilateral salpingo-oophorectomy is considered in postmenopausal women. **Mnemonic:** **SEROUS** — **S**imple cyst, **E**pithelial origin, **R**are malignancy (5–10%), **O**ften <10 cm, **U**nilocular, **S**erous fluid (clear)