What are the different types of breast cancer?

Types of Breast Cancer

Breast cancer is primarily classified based on its origin within breast tissue, with approximately 85-90% of invasive carcinomas being ductal in origin, while the remaining types include various special histological subtypes with distinct clinical behaviors and prognoses. 1

Main Classifications of Breast Cancer

Histological Classification

• Ductal Carcinomas

  • Invasive Ductal Carcinoma (IDC) Not Otherwise Specified (NOS): Most common type (85-90% of cases) 2
  • Ductal Carcinoma In Situ (DCIS): Precancerous condition where abnormal cells are contained within the milk ducts 3

• Lobular Carcinomas

  • Invasive Lobular Carcinoma (ILC): Second most common invasive type
  • Lobular Carcinoma In Situ (LCIS): Considered a risk factor rather than true cancer 1

• Special Histological Subtypes (with generally favorable prognosis)

  • Mucinous (Colloid) Carcinoma: Characterized by mucin production 1, 4
  • Tubular Carcinoma: Well-differentiated with tubule formation 1, 5
  • Medullary Carcinoma: Despite aggressive molecular features, often has better outcomes 5
  • Adenoid Cystic Carcinoma: Rare type with generally good prognosis 5
  • Papillary Carcinoma: Less likely to be hormone receptor negative 4

• Aggressive Special Types

  • Inflammatory Breast Cancer: Clinical presentation with skin changes and rapid progression 1
  • Metaplastic Carcinoma: Contains mixture of epithelial and mesenchymal components 5
  • Micropapillary Carcinoma: Often presents at advanced stage 5

Molecular Classification

Breast cancer is also classified based on molecular characteristics, which significantly impact treatment decisions and prognosis:

• Luminal A: ER+/PR+, HER2-, low Ki-67 (slow growing)
• Luminal B: ER+/PR+, HER2+ or HER2- with high Ki-67 (faster growing)
• HER2-enriched: ER-/PR-, HER2+ (aggressive but responsive to targeted therapy)
• Triple-negative/Basal-like: ER-/PR-, HER2- (generally aggressive) 1, 6

Clinical Characteristics by Histological Type

• Lobular and Ductal/Lobular Carcinomas: More likely to be diagnosed at advanced stages (III/IV), with larger tumors (≥5.0 cm), and node-positive disease compared to pure ductal carcinomas 4

• Mucinous, Tubular, and Papillary Carcinomas: Associated with less aggressive phenotypes, less likely to be hormone receptor negative 4

• Medullary and Inflammatory Carcinomas: More likely to be ER-/PR- negative and high grade (notably, 68.2% of medullary carcinomas are ER-/PR- vs 19.3% of ductal carcinomas) 4

Pathological Assessment

Comprehensive pathology reporting is critical for proper classification and treatment planning, and should include:

• Tumor size (measured in millimeters or centimeters)
• Histological grade (indicates how abnormal cancer cells look compared to normal cells)
• Hormone receptor status (ER/PR)
• HER2 status
• Lymph node involvement 1

Clinical Implications

The histological and molecular classification of breast cancer has significant implications for:

• Prognosis: Special types like mucinous, tubular, and adenoid cystic carcinomas generally have better outcomes than IDC NOS 5

• Treatment approach: Molecular subtypes guide therapy decisions:

  • Hormone receptor-positive: Hormone therapy
  • HER2-positive: Targeted therapy (trastuzumab, lapatinib)
  • Triple-negative: Primarily chemotherapy 1, 6

• Surgical planning: Based on tumor type, size, and molecular characteristics

Important Considerations

• Some histological special types that are associated with good prognosis (like medullary and adenoid cystic carcinomas) may display a poor prognosis basal-like molecular profile, highlighting the complexity of breast cancer classification 5

• Histological special-type cancers are generally homogeneous and belong to one molecular subtype, whereas IDC NOS and ILC contain all molecular breast cancer types 5

• The World Health Organization periodically updates the classification of breast tumors to reflect advances in understanding of breast cancer biology 7