What is the classification of epithelial ovarian tumors?

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Epithelial Ovarian Tumor Classification

Epithelial ovarian tumors are classified according to the WHO 2014/2020 classification system into six major histologic subtypes (serous, mucinous, endometrioid, clear cell, transitional cell/Brenner, and seromucinous), with each subtype further categorized into three prognostically distinct groups: benign, borderline (atypical proliferative), and malignant carcinomas. 1

Major Histologic Subtypes and Their Categories

Serous Tumors

  • Borderline/Atypical Proliferative: Serous borderline tumor/atypical proliferative serous tumor, including micropapillary variant (also termed non-invasive low-grade serous carcinoma) 1
  • Malignant: Low-grade serous carcinoma (LGSC) and high-grade serous carcinoma (HGSC) 1
  • Critical distinction: LGSC and HGSC are fundamentally different disease entities with distinct pathogenesis, molecular alterations (LGSC has KRAS/BRAF mutations; HGSC has TP53 mutations in >80%), and prognosis—not simply different grades of the same tumor 1

Mucinous Tumors

  • Borderline/Atypical Proliferative: Mucinous borderline tumor/atypical proliferative mucinous tumor 1
  • Malignant: Mucinous carcinoma 1
  • Critical pitfall: Must distinguish primary ovarian mucinous tumors from metastatic carcinomas originating in the gastrointestinal tract, pancreas, or cervix 1

Endometrioid Tumors

  • Borderline/Atypical Proliferative: Endometrioid borderline tumor/atypical proliferative endometrioid tumor 1
  • Malignant: Endometrioid carcinoma 1
  • Association: Strongly linked to endometriosis and may indicate Lynch syndrome 1

Clear Cell Tumors

  • Borderline/Atypical Proliferative: Clear cell borderline tumor/atypical proliferative clear cell tumor 1
  • Malignant: Clear cell carcinoma 1
  • Note: These are inherently high-grade tumors, though some publications suggest grading systems, no consensus exists 1

Brenner Tumors (Transitional Cell)

  • Borderline/Atypical Proliferative: Borderline Brenner tumor/atypical proliferative Brenner tumor 1
  • Malignant: Malignant Brenner tumor 1

Seromucinous Tumors

  • Borderline/Atypical Proliferative: Seromucinous borderline tumor/atypical proliferative seromucinous tumor 1
  • Malignant: Seromucinous carcinoma 1

Additional Malignant Categories

  • Undifferentiated carcinoma 1
  • Carcinosarcoma (mixed epithelial-mesenchymal tumor) 1
  • Mixed carcinoma: Only diagnosed when minor component represents ≥10% of the neoplasm (now considered uncommon per WHO 2014) 1

Frequency Distribution

  • High-grade serous carcinoma: 70% of all epithelial ovarian cancers, comprising approximately 80-85% in Western countries 1
  • Endometrioid carcinoma: 10% 1
  • Clear cell carcinoma: 6-10% (particularly common in Japanese women) 1
  • Low-grade serous carcinoma: <5% 1
  • Mucinous carcinoma: 3-4% 1
  • Advanced stage disease (III/IV): Approximately 90% are high-grade serous type, with up to 95% of stage III-IV cases being HGSC 1

Modern Dualistic Pathogenetic Model

Type I Tumors (Indolent)

  • Subtypes: Low-grade serous, mucinous, low-grade endometrioid, clear cell carcinomas 1, 2
  • Characteristics: Arise from well-characterized precursor lesions (borderline tumors), genetically stable, harbor mutations in KRAS, BRAF, PTEN, β-catenin (but NOT TP53) 1, 2
  • Behavior: Slow-growing, often confined to ovary at presentation, relatively good prognosis 2, 3

Type II Tumors (Aggressive)

  • Subtypes: High-grade serous, high-grade endometrioid, undifferentiated carcinoma, carcinosarcoma 1, 2
  • Characteristics: Arise de novo without identifiable precursor, chromosomally unstable, TP53 mutations in >80% 1, 2, 3
  • Behavior: Highly aggressive, rapid evolution, almost always advanced stage at presentation 2, 3

Clinical Implications of Accurate Typing

  • Targeted therapy: Tumor typing is increasingly critical for treatment selection, particularly for PARP inhibitors in BRCA1/2-mutated or HRD-positive high-grade tumors 1
  • Genetic testing: HGSC may indicate BRCA1/2 germline mutations; endometrioid and clear cell carcinomas can occur with Lynch syndrome 1
  • Site of origin: Many tumors traditionally considered ovarian primaries actually originate in the fallopian tube (serous), endometrium (endometrioid/clear cell), or tubal-mesothelial junction (mucinous/Brenner) 1, 3
  • Grading caveat: Different grading systems should be used for different subtypes; universal grading systems are not applicable to all epithelial malignancies 1

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

2
Research

Histologic, molecular, and cytogenetic features of ovarian cancers: implications for diagnosis and treatment.

Radiographics : a review publication of the Radiological Society of North America, Inc, 2011

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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