Breast Cancer Staging: T, M, and G Definitions
T (Tumor) Category
The T category describes the size and local extent of the primary tumor, measured in millimeters, and is determined by either clinical examination/imaging (cT) or pathological assessment (pT), with pathological measurements taking precedence. 1
T classification is based on the largest tumor dimension measured to the nearest millimeter, not on the overall stage, and should be designated with 'c' (clinical) or 'p' (pathological) modifiers to indicate the assessment method 1, 2
T1 tumors are ≤20 mm in greatest dimension, T2 tumors are >20 mm but ≤50 mm, and T3 tumors are >50 mm 2
T4 tumors involve chest wall extension (T4a), skin ulceration/satellite nodules/edema (T4b), both features (T4c), or inflammatory carcinoma (T4d), regardless of tumor size 3, 2
A critical pitfall: pectoralis muscle invasion alone does NOT qualify as T4a—only extension to ribs, intercostal muscles, or serratus anterior muscle constitutes chest wall involvement 3
Another common error: nipple retraction or skin dimpling does NOT automatically indicate T4b—these changes can occur in T1-T3 tumors without upstaging 3
For multiple tumors in the same breast, T stage is based only on the largest tumor, with smaller tumors recorded using the '(m)' modifier 1
M (Metastasis) Category
The M category indicates the presence or absence of distant metastases beyond regional lymph nodes. 1
M0 means no clinical or radiographic evidence of distant metastases 1, 2
M1 indicates distant detectable metastases determined by clinical, radiographic, or histological means with tumor deposits >0.2 mm 1
cM0(i+) is a special designation for molecularly or microscopically detected tumor cells in blood, bone marrow, or non-regional nodes that are ≤0.2 mm in patients without symptoms or signs of metastases 1
G (Grade) Determination
Grade (G) is determined by histopathological examination of tumor tissue and reflects the degree of cellular differentiation and aggressiveness, typically using standardized grading systems like the Nottingham grading system. 1, 4
Grade is assessed by pathologists examining the surgical specimen or biopsy tissue, evaluating nuclear pleomorphism, tubule formation, and mitotic activity 1
The pathology report should include histologic type and grade, with standardized assessment methodologies recommended 1
Grade classification typically includes G1 (well-differentiated), G2 (moderately differentiated), and G3 (poorly differentiated), with higher grades indicating more aggressive tumors 5
Grade is an independent prognostic factor that, along with ER/PR status, HER2 status, and anatomic extent, determines the final prognostic stage group in the AJCC 8th edition staging system 4
Important Distinctions in Terminology
The terms "T category," "N category," and "M category" should be used—NOT "T stage," "N stage," or "M stage"—because the term "stage" is reserved for the Roman numeral aggregated stage groups (I, II, III, IV). 1
Clinical classification (cTNM) is based on pretreatment evidence from physical examination, imaging, and endoscopy 1
Pathological classification (pTNM) incorporates both clinical and surgical/pathological findings and cannot be determined by the pathologist alone without full clinical information 1
The original stage designation at diagnosis must remain unchanged in medical records, even if anatomic disease extent changes during treatment 1