What is the classification and management of breast tumors in an adult patient with no significant medical history?

Classification of Breast Tumours

Primary Histological Classification

The World Health Organization (WHO) classification system is the standard for histological classification of breast cancer, which categorizes tumours into invasive ductal carcinoma not otherwise specified (IDC NOS) and approximately 17 distinct histological special types. 1, 2

Major Categories

In Situ Carcinomas

Ductal Carcinoma In Situ (DCIS)

• DCIS should be classified primarily by nuclear grade and presence/absence of necrosis, rather than solely by architectural pattern. 1
• The pathology report must clearly specify:
  • Nuclear grade (low, intermediate, or high) 1
  • Presence or absence of necrosis and cell polarization 1
  • Architectural patterns (comedo, cribriform, micropapillary, papillary, solid) 1
• High nuclear grade and extensive comedonecrosis are associated with higher risk of early local recurrence following breast-conserving therapy 1
• Micropapillary subtypes tend to be more extensive and prone to multiple quadrant involvement 1

Lobular Carcinoma In Situ (LCIS)

• LCIS is considered a marker of increased risk for subsequent breast cancer rather than a malignant lesion requiring surgical excision 1
• The increased risk applies to both breasts and is lifelong 1
• Surgical margin status is not relevant for LCIS 1

Invasive Carcinomas

Invasive Ductal Carcinoma Not Otherwise Specified (IDC NOS)

• Comprises approximately 75% of invasive breast cancers 2
• Should be graded using the Elston and Ellis histoprognostic grading system 1
• Contains all molecular breast cancer subtypes (luminal, basal-like, HER2+) 2

Histological Special Types (Favorable Prognosis)

• To be classified as favorable histology, the tumor must be pure (>90% on surgical excision, not core biopsy alone), not high grade, and HER2-negative. 1
• Include:
  • Pure tubular carcinoma 1, 2
  • Pure mucinous carcinoma (types A and B) 1, 2
  • Pure cribriform carcinoma 1, 2
  • Adenoid cystic carcinoma 1, 2
  • Secretory carcinoma 1
  • Neuroendocrine carcinoma 2
• The vast majority are ER-positive and HER2-negative 1
• If ER-negative or HER2-positive results are reported for these favorable histologies, the pathology evaluation and receptor determination accuracy should be reviewed. 1

Other Special Types

• Invasive lobular carcinoma (ILC) 1, 2
• Micropapillary carcinoma 2
• Apocrine carcinoma 2
• Medullary carcinoma 2
• Metaplastic carcinoma (including low-grade variants like adenosquamous and fibromatosis-like) 1, 2

Non-Epithelial Tumours

Phyllodes Tumours

• Classified as benign, borderline (intermediate malignancy), or malignant 3
• Differential diagnosis with highly proliferative fibroadenoma can be difficult 3
• Prognosis determined by histological subtype, type of surgery, and stromal proliferation 3
• Risk of local relapse and distant metastases (particularly lung) 3

Breast Sarcomas

• All histological types can occur, with predominance of histiocytofibroma-type tumours 3
• Angiosarcomas characterized by high risk in irradiated fields and poor prognosis 3, 4
• All breast sarcomas and malignant phyllodes tumours should be referred to specialist sarcoma centers for pathology review and multidisciplinary discussion. 4

Critical Pathological Reporting Elements

For DCIS

• Surgical margin assessment is the most important aspect of pathologic evaluation for DCIS. 1
• Report must specify whether DCIS is transected at the surgical margin and, if not, the distance from the nearest margin 1
• Tumor extent (size) should be estimated using one of four methods: direct measurement on single slide, sequential examination of entire specimen, percentage of breast tissue involved, or number of slides with DCIS out of total examined 1

For Invasive Carcinomas

• Tumor size (extent of invasive component) 1
• Histological grade (Elston and Ellis system) 1
• Lymph node status (at least 10 lymph nodes should be examined for optimal quality) 1
• Surgical margin status 1
• ER/PR receptor status 1
• HER2 status 1

Molecular Classification Integration

While histological classification remains fundamental, molecular subtyping (luminal A, luminal B, HER2-enriched, basal-like) increasingly guides treatment decisions. 2, 5

• Most histological special types (except apocrine carcinoma) are homogeneous and belong to only one molecular subtype 2
• IDC NOS and ILC contain all molecular breast cancer types 2
• Some special types with good prognosis (medullary, adenoid cystic) display basal-like transcriptome despite favorable outcomes, indicating heterogeneity within molecular subtypes 2

Common Pitfalls to Avoid

• Do not perform frozen section examination on image-guided needle biopsies of nonpalpable lesions or mammographically directed biopsies for microcalcifications, as distinguishing atypical ductal hyperplasia from DCIS may be impossible and small foci of microinvasion may be lost 1
• Do not classify favorable histology special types based on core biopsy alone; require surgical excision showing >90% pure histology 1
• If atypical pathologic or clinical features are present in special types, treat as ductal/no special type. 1
• Do not confuse pectoralis muscle invasion with chest wall extension for T4 staging 6