A 62-year-old woman undergoes laparotomy for a right ovarian mass. Histology shows a papillary serous cystadenoma with nuclear atypia and 1 mitosis per 10 HPF but without stromal invasion. What is the WHO 2020 classification of this tumour and its typical clinical behaviour?

• A Low-grade serous ovarian carcinoma — chemosensitive, generally responds to platinum
• B Serous borderline tumour (SBT) — low malignant potential, with 5-year survival >90% even when extra-ovarian implants are present ✓
• C High-grade serous carcinoma — aggressive, treated with cytoreductive surgery and carboplatin/paclitaxel
• D Serous cystadenoma — benign, requires only cystectomy

Explanation

Serous borderline tumours (also called atypical proliferative serous tumours per WHO 2020) are characterized by epithelial stratification, nuclear atypia, and mitotic activity WITHOUT destructive stromal invasion. They account for ~15% of serous ovarian tumours and have an indolent course; 5-year survival exceeds 95% for stage I and remains >70–80% even with non-invasive peritoneal implants (stage III). Invasive implants or micropapillary pattern do worsen prognosis. Low-grade serous carcinoma (LGSC) does show stromal invasion and is relatively platinum-resistant. High-grade serous carcinoma has p53 mutation and TP53 aberration as hallmarks.

Reference: Shaw's Textbook of Gynaecology, 17th ed.

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