TNM Staging System for Colorectal Cancer
The TNM staging system is the recommended standard for colorectal cancer staging, providing essential prognostic information for treatment decisions by evaluating tumor penetration depth (T), lymph node status (N), and presence of distant metastases (M). 1
T (Tumor) Classification
- T1: Tumor invades submucosa 2
- T2: Tumor invades muscularis propria 2
- T3: Tumor invades through muscularis propria into subserosa or into non-peritonealised pericolic or perirectal tissues 2
- T4: Tumor directly invades other organs/structures and/or perforates visceral peritoneum 2
N (Node) Classification
- N0: No regional lymph node metastasis 1
- N1: Metastasis in 1-3 regional lymph nodes 1
- N1a: Metastasis in 1 regional lymph node
- N1b: Metastasis in 2-3 regional lymph nodes
- N1c: Tumor deposits in subserosa, mesentery, or non-peritonealised pericolic/perirectal tissues without regional nodal metastasis 2
- N2: Metastasis in 4 or more regional lymph nodes 1
- N2a: Metastasis in 4-6 regional lymph nodes
- N2b: Metastasis in 7 or more regional lymph nodes 1
M (Metastasis) Classification
- M0: No distant metastasis 1
- M1: Distant metastasis 1
- M1a: Metastasis confined to one organ without peritoneal metastasis
- M1b: Metastasis in more than one organ
- M1c: Metastasis to peritoneum with or without other organ involvement 2
Stage Grouping
- Stage I: T1-2, N0, M0 3
- Stage II: T3-4, N0, M0 3
- Stage IIA: T3, N0, M0
- Stage IIB: T4a, N0, M0
- Stage IIC: T4b, N0, M0 1
- Stage III: Any T, N1-2, M0 3
- Stage IIIA: T1-2, N1, M0 or T1, N2a, M0
- Stage IIIB: T3-4a, N1, M0 or T2-3, N2a, M0 or T1-2, N2b, M0
- Stage IIIC: T4a, N2a, M0 or T3-4a, N2b, M0 or T4b, N1-2, M0 1
- Stage IV: Any T, Any N, M1 1
- Stage IVA: Any T, Any N, M1a
- Stage IVB: Any T, Any N, M1b
- Stage IVC: Any T, Any N, M1c 1
Diagnostic Considerations for Staging
- Preoperative staging should include clinical examination, blood counts, liver and renal function tests, CEA, chest imaging, abdominal CT scan, and complete colonoscopy 3, 1
- Pelvic MRI is the most accurate test for locoregional staging of rectal cancer, detecting extramural vascular invasion and determining distance to the circumferential resection margin 3
- Endoscopic rectal ultrasound (ERUS) is valuable for early tumors but less useful in locally advanced rectal cancer 3
Clinical Implications of TNM Staging
- Pathologic assessment should include examination of at least 12 lymph nodes to prevent understaging 1
- Adjuvant chemotherapy is recommended for stage III (T1-4, N1-2, M0) and may be considered in selected high-risk stage II patients 3, 1
- High-risk factors in stage II include: T4 tumors, poor differentiation, vascular/lymphatic/perineural invasion, obstruction or perforation, examination of fewer than 12 lymph nodes, and elevated CEA 3
- Tumor budding should be included among high-risk factors reported in stage II colorectal cancer 3
Common Pitfalls in TNM Staging
- Inadequate lymph node sampling (<12 nodes) can lead to understaging and inappropriate treatment decisions 1
- Mistaking inflammatory adhesion for tumor invasion can lead to incorrect T classification 2
- Confusing peritoneal involvement through direct continuity (T4a) with discontinuous peritoneal deposition (M1c) can result in incorrect staging 2
- Tumor deposits in subserosa without regional lymph node metastasis should be classified as N1c, not affecting the T category 2
The TNM staging system continues to evolve with each edition to better reflect patient prognosis and guide treatment decisions, with the most recent being the 8th edition from the UICC/AJCC 2, 4.