WHO Classification of Breast Tumours - 6th Edition (2026)

Important correction: The 6th edition was released in April 2026 as an online beta version on the WHO Tumour Classification Online platform (prepared by 112 authors/editors, ~2,900 references). The 5th edition was published in 2022. This review is based on the 6th edition per the Quinn et al., Histopathology 2026 summary paper (PMID 42011085).

Full Classification (ICD-O Codes)

1. Benign Epithelial Proliferations and Precursors

• Usual ductal hyperplasia
• Columnar cell lesions including flat epithelial atypia (FEA)
• Atypical ductal hyperplasia (ADH)

2. Noninvasive Lobular Neoplasia

• Atypical lobular hyperplasia (ALH)
• Lobular carcinoma in situ (LCIS), NOS 8520/2
  • Classic LCIS
  • Florid LCIS
  • Pleomorphic LCIS 8519/2

3. Ductal Carcinoma In Situ (DCIS)

• Ductal carcinoma, noninfiltrating, NOS 8500/2
  • DCIS of low nuclear grade
  • DCIS of intermediate nuclear grade
  • DCIS of high nuclear grade

4. Invasive Breast Carcinoma

• Invasive carcinoma of no special type (IBC-NST) 8500/3
  • Pleomorphic carcinoma
  • Carcinoma with osteoclast-like stromal giant cells
  • Carcinoma with choriocarcinomatous features
  • IBC-NST with medullary pattern (replaces "medullary carcinoma")
  • IBC-NST with basal-like features
• Invasive lobular carcinoma (ILC) 8520/3
  • Classic ILC
  • Solid ILC
  • Alveolar ILC
  • Pleomorphic ILC
  • Tubulo-lobular ILC
  • Mixed ILC
  • ILC with extracellular mucin - NEW entity in 6th edition
• Tubular carcinoma 8211/3
• Cribriform carcinoma 8201/3
• Mucinous carcinoma 8480/3 (now restricted to grade 1/2, favorable biomarker profile)
• Carcinoma with apocrine differentiation 8401/3
• Invasive micropapillary carcinoma 8507/3
• Metaplastic carcinoma 8575/3
  • Low-grade adenosquamous carcinoma
  • Fibromatosis-like metaplastic carcinoma
  • Squamous cell carcinoma
  • Spindle cell carcinoma
  • Metaplastic carcinoma with heterologous mesenchymal differentiation
  • Mixed metaplastic carcinoma
• Mucinous cystadenocarcinoma 8470/3
• Polymorphous adenocarcinoma 8525/3

5. Rare and Salivary Gland-Type Tumours

• Secretory carcinoma 8502/3
• Acinar cell carcinoma 8550/3
• Mucoepidermoid carcinoma 8430/3
• Adenoid cystic carcinoma 8200/3
  • Classic adenoid cystic carcinoma
  • Solid-basaloid adenoid cystic carcinoma
  • Adenoid cystic carcinoma with high-grade transformation
• Tall cell carcinoma with reversed polarity 8509/3

6. Epithelial-Myoepithelial Tumours

• Pleomorphic adenoma 8940/0
• Adenomyoepithelioma, NOS 8983/0
• Adenomyoepithelioma with carcinoma 8983/3
• Epithelial-myoepithelial carcinoma 8562/3

7. Neuroendocrine Neoplasms (revised classification)

• Neuroendocrine tumor (NET) grade 1 8240/3
• Neuroendocrine tumor (NET) grade 2 8249/3
• Neuroendocrine carcinoma (NEC), NOS 8246/3
  • Small cell neuroendocrine carcinoma 8041/3
  • Large cell neuroendocrine carcinoma 8013/3

8. Papillary Tumours

• Intraductal papilloma 8503/0
• Intraductal papillary carcinoma (papillary DCIS) 8503/2
• Encapsulated papillary carcinoma (EPC) 8504/2
  • EPC with invasion 8504/3
• Solid papillary carcinoma (SPC) in situ 8509/2
• Invasive solid papillary carcinoma 8509/3

9. Fibroepithelial Tumours

• Fibroadenoma, NOS 9010/0
• Phyllodes tumor, benign 9020/0
• Phyllodes tumor, borderline 9020/1
• Phyllodes tumor, malignant 9020/3
• Periductal stromal tumor
• Hamartoma

10. Tumours of the Nipple

• Nipple adenoma 8506/0
• Syringoma 8407/0
• Paget disease of the nipple 8540/3

11. Mesenchymal Tumours

• Hemangioma types (perilobular, venous, cavernous, capillary) 9120/0
• Angiomatosis
• Atypical vascular lesion 9126/0 (lymphatic / vascular types)
• Angiosarcoma 9120/3 (including post-radiation)

• Nodular fasciitis 8828/0
• Myofibroblastoma 8825/0
• Desmoid-type fibromatosis 8821/1
• Inflammatory myofibroblastic tumor 8825/1

• Leiomyoma 8890/0 (cutaneous; nipple/areola)
• Leiomyosarcoma 8890/3

• Lipoma 8850/0
• Angiolipoma 8861/0
• Liposarcoma 8850/3

• Schwannoma 9560/0
• Neurofibroma 9540/0
• Granular cell tumor 9580/0 / malignant 9580/3

• Pseudoangiomatous stromal hyperplasia (PASH)

12. Malignant Lymphoma

• Diffuse large B-cell lymphoma, NOS 9680/3
• Burkitt lymphoma 9687/3 (endemic, sporadic, immunodeficiency-associated)
• Breast implant-associated anaplastic large cell lymphoma (BIA-ALCL) 9715/3
• Mucosa-associated lymphoid tissue (MALT) lymphoma 9699/3
• Follicular lymphoma 9690/3

13. Metastatic Tumours

14. Tumours of the Male Breast

• Gynecomastia
• Invasive carcinoma 8500/3
• In situ carcinoma 8500/2

Key Changes in the 6th Edition (2026) vs. 5th Edition (2022)

HER2 Reporting - Major Update

• HER2-positive (IHC 3+ or ISH-amplified)
• HER2-low (IHC 1+ or IHC 2+/ISH-non-amplified)
• HER2-ultralow (IHC 0 with faint/incomplete membrane staining in >0% but ≤10% of cells) - newly defined; eligible for trastuzumab deruxtecan (T-DXd)
• HER2-null/zero (IHC 0 with no membrane staining at all) The term 'variant' is now reserved exclusively for molecular/genetic alterations (not morphologic subtypes).

New Diagnostic Entity

• ILC with extracellular mucin - recognized as a distinct subtype with specific prognostic implications.

Mucinous Carcinoma - Restricted Definition

• Diagnosis now restricted to mucin-secreting tumors with grade 1 or 2 morphology and a favorable biomarker profile. Higher-grade mucinous tumors fall under IBC-NST.

Phyllodes Tumour - Revised Criteria for Malignancy

• Malignant phyllodes now requires only 4 of the original 5 adverse histological criteria (liposarcomatous differentiation alone is no longer sufficient).

Neuroendocrine Tumours - Revised Classification

• The unified model from the 5th edition proved difficult to apply; classification is revised to better reflect breast-specific neuroendocrine biology.

Solid Papillary Carcinoma (SPC) - Clarification

• SPC is now clearly classified as SPC in situ vs. invasive SPC, resolving prior ambiguity.

New Sections Added

1. "Small Diagnostic Samples" - covers non-operative biopsy, the B-coding system, and multidisciplinary review.
2. "Digital Pathology" - covers AI-assisted diagnostics, advantages, and challenges.

Mitotic Count Standardization

• Mitotic counts are now expressed as per mm² (instead of per 10 HPF) to account for variation in microscope field sizes.

Summary of Classification Hierarchy