What are the different types of breast cancer?

Types of Breast Cancer

Breast cancer is primarily classified into non-invasive (in situ) and invasive carcinomas, with invasive ductal carcinoma (70-75%) and invasive lobular carcinoma (12-15%) being the most common types. 1

Non-Invasive (In Situ) Carcinomas

• Ductal Carcinoma In Situ (DCIS)

  • Abnormal epithelial cells confined within the breast ducts
  • Considered a non-lethal precursor to invasive breast cancer 2
  • When treated with breast-conserving surgery, all subgroups benefit from adjuvant radiation 1
  • Tamoxifen is indicated for ER-positive DCIS but may be detrimental in ER-negative disease 1

• Lobular Carcinoma In Situ (LCIS)

  • Risk factor for future development of invasive cancer
  • Should be completely resected 1
  • Not considered a true cancer but a marker for increased breast cancer risk

Invasive Carcinomas

Major Types:

1. Invasive Ductal Carcinoma Not Otherwise Specified (IDC NOS)

   • Most common type (70-75% of all breast cancers)
   • Originates from the ductal epithelium
   • Can display any molecular subtype (luminal, basal-like, HER2+) 3

2. Invasive Lobular Carcinoma (ILC)

   • Second most common type (12-15%)
   • Originates from the lobular epithelium
   • Like IDC, can display various molecular subtypes 3

Special Histological Types (collectively ~18% of breast cancers):

• Tubular Carcinoma

  • Well-differentiated with tubule formation
  • Generally good prognosis

• Mucinous Carcinoma (Types A and B)

  • Characterized by extracellular mucin production
  • Usually has favorable prognosis

• Medullary Carcinoma

  • Well-circumscribed with lymphocytic infiltration
  • Despite basal-like molecular features, often has better prognosis than expected 3

• Neuroendocrine Carcinoma

  • Shows neuroendocrine differentiation
  • No specific prognostic or therapeutic implications 1

• Micropapillary Carcinoma

  • Constitutes a discrete entity at the transcriptome level 3
  • Often associated with lymphovascular invasion

• Adenoid Cystic Carcinoma

  • Rare type with generally good prognosis despite basal-like features 3

• Metaplastic Carcinoma

  • Contains squamous, spindle cell, or heterologous elements
  • Often aggressive behavior

• Inflammatory Breast Cancer

  • Clinical diagnosis rather than histological type
  • Classified as T4d in TNM staging 1
  • Characterized by erythema, edema, and peau d'orange appearance
  • Poor prognosis

Molecular Classification

Modern breast cancer management increasingly incorporates molecular classification alongside histological typing 1:

1. Luminal A-like

   • ER/PR positive, HER2 negative, low Ki67
   • Generally good prognosis

2. Luminal B-like

   • ER positive, may be HER2 positive or negative
   • Higher Ki67 than Luminal A
   • More aggressive than Luminal A

3. HER2-enriched

   • HER2 overexpression/amplification
   • ER/PR negative
   • Targeted therapy with anti-HER2 agents

4. Triple-negative/Basal-like

   • ER, PR, and HER2 negative
   • Generally aggressive behavior
   • Heterogeneous group with varying outcomes 3

Clinical Implications

• While histological special types like tubular and lobular carcinomas may appear distinct morphologically, some are very similar at the transcriptome level 3

• Most histological special types are homogeneous and belong to only one molecular subtype, whereas IDC NOS and ILC contain all molecular breast cancer types 3

• Some special types with traditionally good prognosis (like medullary and adenoid cystic carcinomas) display basal-like transcriptomes, suggesting that basal-like cancers are heterogeneous in behavior 3

• Recent research suggests that cancers originating from major lactiferous ducts (termed Ductal Adenocarcinoma of the Breast or DAB) may be underdiagnosed as DCIS with microinvasion despite having poor outcomes 4

• Similarly, cancers originating from terminal ductal lobular units (TDLUs) have been proposed to be termed Acinar Adenocarcinoma of the Breast (AAB) to better reflect their origin 5

Understanding the specific type of breast cancer is crucial for determining appropriate treatment strategies and predicting outcomes, with molecular classification increasingly guiding therapeutic decisions alongside traditional histopathological assessment.