TNM Staging System for Breast Cancer
The TNM staging system for breast cancer evaluates three key components: Tumor size and extent (T), regional lymph Node involvement (N), and distant Metastasis (M), with the most current eighth edition incorporating prognostic biomarkers alongside anatomic staging to better predict outcomes and guide treatment decisions. 1, 2, 3
Core TNM Components
T Classification (Primary Tumor)
The T category describes tumor size and local extension 1, 2:
- Tis: Carcinoma in situ (DCIS, LCIS, or Paget's disease without underlying invasive cancer) 1
- T1: Tumor ≤20 mm, subdivided into:
- T2: Tumor >20 mm but ≤50 mm 1
- T3: Tumor >50 mm 1
- T4: Any size with chest wall extension and/or skin involvement (ulceration, satellite nodules, peau d'orange, or inflammatory carcinoma) 1
N Classification (Regional Lymph Nodes)
The number of involved lymph nodes critically impacts staging and prognosis. 1
Key Nodal Staging Criteria:
- pN0(i+): Isolated tumor cells ≤0.2 mm detected by H&E or IHC 1
- pN1mi: Micrometastases >0.2 mm but ≤2.0 mm 1
- pN1: 1-3 positive axillary lymph nodes, OR internal mammary nodes detected only by sentinel node biopsy 1
- pN2: 4-9 positive axillary lymph nodes, OR clinically detected internal mammary nodes without axillary involvement, OR infraclavicular nodes 1
- pN3: ≥10 positive axillary lymph nodes, OR ipsilateral supraclavicular nodes, OR internal mammary nodes with concurrent axillary involvement 1
Critical change from prior editions: Ipsilateral supraclavicular lymph node metastases are now classified as N3 disease rather than M1 (distant metastasis), creating the new Stage IIIC category for T any, N3, M0 disease. 1
M Classification (Distant Metastasis)
Staging Workup Requirements
Minimum Assessment
All patients require full blood count, liver and renal function tests, alkaline phosphatase, and calcium levels before surgery and systemic therapy. 1
Extended Staging Indications
Perform CT chest, abdominal imaging (US/CT/MRI), and bone scan for patients with 1:
- Clinically positive axillary nodes
- Large tumors (≥5 cm)
- Aggressive biology
- Clinical signs, symptoms, or laboratory abnormalities suggesting metastases
PET-CT can replace traditional imaging in high-risk patients but has limited sensitivity for locoregional disease compared to sentinel lymph node biopsy. 1
Pathologic Assessment Requirements
The surgical specimen evaluation must include 1, 2:
- Number, location, and maximum diameter of all tumors removed
- Total number of lymph nodes removed and number positive
- Extent of lymph node metastases (isolated tumor cells, micrometastases 0.2-2 mm, or macrometastases)
- Histological type and grade using standardized grading systems
- Resection margin status with location and minimum distance
- Vascular invasion presence
- Mandatory biomarker analysis: ER, PR, and HER2 status 1, 2
Eighth Edition Updates (2018)
The eighth edition represents a paradigm shift by incorporating prognostic biomarkers (tumor grade, hormone receptor status, HER2 expression, and multigene panel scores) with anatomic staging to create prognostic stage groups that better predict individual outcomes. 3
This integration allows for more accurate risk stratification beyond anatomic extent alone, particularly for ER-positive, lymph node-negative tumors where genomic assays may downstage disease 1, 3.
Common Pitfalls
Incomplete pathology reporting occurs in up to 50% of breast cancer reports, with critical omissions including failure to orient and report surgical margins and inconsistent tumor grading. 1
For small tumors diagnosed by core biopsy, measuring only residual tumor in the excision specimen results in understaging; correlation of imaging, clinical, and gross findings with microscopic observation is essential. 1
Sentinel lymph node biopsy with IHC evaluation has increased detection of micrometastases and isolated tumor cells, requiring specific identifiers in staging to indicate detection methods used. 1