TNM Staging System for Sigmoid Colon Adenocarcinoma
The sigmoid colon is staged using the UICC TNM classification system (8th edition, 2017), which is identical to the staging system for all colon cancers, with T stage based on depth of invasion, N stage on number of involved regional lymph nodes, and M stage on presence of distant metastases. 1
T Stage (Primary Tumor Depth of Invasion)
The T classification describes how deeply the tumor has invaded through the bowel wall layers 1:
- Tis: Carcinoma in situ—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
- T2: Tumor invades muscularis propria 1
- T3: Tumor invades through the muscularis propria into the subserosa or into non-peritonealized pericolic tissues 1
- T4a: Tumor perforates the visceral peritoneum 1
- T4b: Tumor directly invades other organs or structures 1
Important caveat: Unlike rectal cancer, sigmoid colon tumors do not require T3 sub-classification by depth of extramural invasion, as this distinction is primarily relevant for rectal cancer treatment planning. 2
N Stage (Regional Lymph Node Involvement)
The N classification is based on the number of involved regional lymph nodes, not their size or location 1:
- N0: No regional lymph node metastasis 1
- N1a: Metastasis in 1 regional lymph node 1
- N1b: Metastasis in 2-3 regional lymph nodes 1
- N1c: Tumor deposits (satellites) in the subserosa or non-peritonealized pericolic soft tissue without regional lymph node metastasis 1
- N2a: Metastasis in 4-6 regional lymph nodes 1
- N2b: Metastasis in 7 or more regional lymph nodes 1
Critical pitfall: N1c should only be used when there are tumor deposits but no positive lymph nodes; if any lymph nodes are positive, stage according to the number of positive nodes and document deposits separately. 1, 3
M Stage (Distant Metastasis)
The M classification describes the presence and extent of distant metastatic disease 1:
- M0: No 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 Groupings
The TNM categories are combined into overall stages that guide treatment and predict prognosis 4, 5:
- Stage 0: Tis, N0, M0
- Stage I: T1-2, N0, M0 (5-year survival ~93%) 4
- Stage IIA: T3, N0, M0 (5-year survival ~85%) 4
- Stage IIB: T4a, N0, M0 (5-year survival ~72%) 4
- Stage IIC: T4b, N0, M0 4
- Stage IIIA: T1-2, N1/N1c, M0 or T1, N2a, M0 (5-year survival ~83%) 4
- Stage IIIB: T3-4a, N1/N1c, M0 or T2-3, N2a, M0 or T1-2, N2b, M0 (5-year survival ~64%) 4
- Stage IIIC: T4a, N2a, M0 or T3-4a, N2b, M0 or T4b, N1-2, M0 (5-year survival ~44%) 4
- Stage IV: Any T, Any N, M1 (5-year survival ~8%) 4
Pathological Requirements for Accurate Staging
At least 12 lymph nodes must be examined to accurately stage sigmoid colon cancer and prevent understaging. 2, 4, 5 Fewer than 12 nodes is a recognized quality metric failure that can lead to inappropriate omission of adjuvant chemotherapy in node-negative patients who may actually have occult N1 disease. 4, 5
Additional pathological features that must be documented include 2:
- Circumferential resection margins (though less critical than in rectal cancer)
- Vascular invasion (lymphatic and venous)
- Perineural invasion
- Tumor deposits (if present)
- Histologic grade
Key distinction from rectal cancer: Sigmoid colon tumors are defined as those located >15 cm from the anal verge by rigid sigmoidoscopy; tumors ≤15 cm are classified as rectal and staged differently with additional considerations for mesorectal fascia involvement and circumferential resection margin. 6, 2