What is the TNM (Tumor, Node, Metastasis) staging system for rectal cancer?

TNM Staging System for Rectal Cancer

The TNM staging system should be used for rectal cancer, with the UICC TNM 8th edition (2017) being the most current classification, though version 7 (2010) remains acceptable. 1

Definition of Rectal Cancer

• Tumors with distal extension to ≤15 cm from the anal margin (as measured by rigid sigmoidoscopy) are classified as rectal; more proximal tumors are classified as colonic. 1

T Stage (Primary Tumor)

The T classification describes the depth of tumor invasion through the rectal wall:

• Tis: Cancer cells confined within the mucosal lamina propria (intramucosal) with no extension through the muscularis mucosae into the submucosa 1
• T1: Tumor invades submucosa 1
  • Can be further subclassified using Haggitt's system (for pedunculated lesions) or Kudo/Kikuchi sm-system (for sessile lesions) to predict lymph node metastasis risk 1
• T2: Tumor invades muscularis propria 1
• T3: Tumor invades through the muscularis propria into pericolorectal tissues 1
  • Critical subclassification based on depth of extramural invasion beyond muscularis propria: T3a (<1 mm), T3b (1-5 mm), T3c (6-15 mm), T3d (>15 mm) 1
  • This subclassification is clinically valuable for treatment decisions and should be used in MRI staging 1
• T4a: Tumor invades through visceral peritoneum to involve the surface 1
• T4b: Tumor directly invades other organs or structures 1
  • Tumor adherent to other organs macroscopically is classified cT4b, but if no tumor is present in the adhesion microscopically, classification should be pT1-3 depending on depth of wall invasion 1

N Stage (Regional Lymph Nodes)

• N0: No regional lymph node metastasis 1
• N1: Metastasis in 1-3 regional lymph nodes 1
  • N1a: Metastasis in 1 regional lymph node 1
  • N1b: Metastasis in 2-3 regional lymph nodes 1
  • N1c: Tumor deposits (satellites) in pericolorectal adipose tissue without regional lymph node metastasis 1
• N2: Metastasis in 4 or more regional lymph nodes 1
  • N2a: Metastasis in 4-6 regional lymph nodes 1
  • N2b: Metastasis in 7 or more regional lymph nodes 1

Important caveat: Nodal staging is very unreliable even using ERUS, CT, and MRI combined; node size >10 mm alone is inaccurate, and irregular border with heterogeneous signal provides more relevant information. 1

M Stage (Distant Metastasis)

• M0: No distant metastasis 1
• M1: Distant metastasis 1
  • M1a: Metastasis confined to one organ (liver, lung, ovary, non-regional lymph nodes) without peritoneal metastases 1
  • M1b: Metastasis in more than one organ 1
  • M1c: Metastasis to the peritoneum with or without other organ involvement 1

Stage Grouping

Based on the AJCC/UICC system: 2

• Stage I: T1-2, N0, M0
• Stage II: T3-4, N0, M0
• Stage III: Any T, N1-2, M0
• Stage IV: Any T, Any N, M1

Staging Modalities

Clinical Staging

• Pelvic MRI is the most accurate test for locoregional staging, detecting extramural vascular invasion (EMVI), determining T substage, and measuring distance to circumferential resection margin (CRM). 1, 2
• Endoscopic rectal ultrasound (ERUS) is valuable for earliest tumors (cT1-T2) to determine if lesions are limited to mucosa or submucosa, but offers less value in locally advanced disease. 1

Pathological Staging Requirements

• Examination of at least 12 regional lymph nodes is required to accurately stage and prevent understaging. 1, 2
• Assessment of circumferential resection margin (CRM) status is critical. 1
• Evaluation of vascular and perineural invasion should be performed. 1

Common Pitfalls

• Inadequate lymph node sampling (<12 nodes) leads to understaging and inappropriate treatment decisions. 2
• Tumor deposits are discrete nodules in pericolorectal adipose tissue without identifiable lymph node structure; if vessel wall or neural structures are identifiable, they should be classified as vascular (V1/2) or perineural invasion (Pn1) instead. 1
• Version controversy: While TNM 8th edition is current, some earlier versions (particularly version 5 from 1997) were preferred in certain contexts due to less interobserver variation in defining stage II and III disease. 1