A 58-year-old woman undergoes staging laparotomy for a newly diagnosed ovarian mass. Intraoperative frozen section confirms high-grade serous ovarian cancer. Which single feature best distinguishes high-grade serous ovarian cancer (HGSOC) from low-grade serous ovarian cancer (LGSOC)?

Explanation

## Distinguishing High-Grade from Low-Grade Serous Ovarian Cancer ### Molecular and Histologic Hallmarks **Key Point:** TP53 mutations (present in ~95% of HGSOC but rare in LGSOC) combined with high mitotic rate (>10 mitoses/10 hpf) are the defining features that distinguish HGSOC from LGSOC. These represent fundamentally different molecular pathways and disease biology. ### Comparative Feature Table | Feature | High-Grade Serous (HGSOC) | Low-Grade Serous (LGSOC) | | --- | --- | --- | | **TP53 mutations** | ~95% (pathognomonic) | <5% (rare) | | **KRAS/BRAF mutations** | Rare | Common (~70%) | | **Mitotic rate** | High (>10/10 hpf) | Low (<10/10 hpf) | | **Nuclear grade** | Grade 3 (severe atypia) | Grade 1–2 (mild–moderate atypia) | | **Age at presentation** | 55–65 years (peak 6th decade) | 40–50 years (peak 5th decade) | | **Stage at diagnosis** | 60–70% stage III–IV | 70% stage I–II | | **Prognosis** | Poor (5-year OS ~40%) | Better (5-year OS ~80%) | | **CA-125** | Markedly elevated | Mildly to moderately elevated | | **Laterality** | Often bilateral | Usually unilateral | ### Molecular Basis **High-Yield:** HGSOC arises via a "de novo" pathway with TP53 mutations as an early event, leading to rapid progression and aggressive behavior. LGSOC arises via a "stepwise" pathway from borderline tumors, acquiring KRAS or BRAF mutations, and progresses slowly. This molecular distinction is now recognized as critical for prognosis and treatment selection (HGSOC responds to platinum–taxane chemotherapy; LGSOC is chemotherapy-resistant and may benefit from targeted therapy). ### Clinical Pearl **Clinical Pearl:** On frozen section, the pathologist will report nuclear grade and mitotic rate. The presence of severe nuclear atypia with brisk mitotic activity (high-grade) is the intraoperative clue that distinguishes HGSOC from LGSOC. Molecular testing for TP53 mutations confirms the diagnosis but is not always available intraoperatively. ### Mnemonic **Mnemonic:** **TP53-HG** = TP53 mutations + High-grade = HGSOC. LGSOC = Low-grade + KRAS/BRAF. [cite:Robbins 10e Ch 24]