TNM Staging System for Rectal Cancer

Definition and Staging

The Union for International Cancer Control (UICC) recommends using the TNM staging system for rectal cancer, with the 8th edition (2017) being the most current classification, though version 7 (2010) remains acceptable 1, 2
Tumors with distal extension to ≤15 cm from the anal margin are classified as rectal; more proximal tumors are classified as colonic 3, 4, 5

The T classification describes the depth of tumor invasion through the rectal wall, with Tis being cancer cells confined within the mucosal lamina propria, T1 invading submucosa, T2 invading muscularis propria, T3 invading through the muscularis propria into pericolorectal tissues, and T4a and T4b indicating more extensive invasion 1, 2
The T3 classification can be further subclassified based on depth of extramural invasion beyond muscularis propria: T3a (<1 mm), T3b (1-5 mm), T3c (6-15 mm), T3d (>15 mm) 1, 2

The N classification describes the extent of regional lymph node metastasis, with N0 indicating no metastasis, N1 indicating metastasis in 1-3 regional lymph nodes, and N2 indicating metastasis in 4 or more regional lymph nodes 1
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 2

The M classification describes the presence or absence of distant metastasis, with M0 indicating no distant metastasis and M1 indicating distant metastasis 1
M1 can be further subclassified as M1a (metastasis confined to one organ), M1b (metastasis in more than one organ), and M1c (metastasis to the peritoneum with or without other organ involvement) 1

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 5, 1, 2

Examination of at least 12 regional lymph nodes is required to accurately stage and prevent understaging 3, 4, 5
Assessment of circumferential resection margin (CRM) status is critical, as well as evaluation of vascular and perineural invasion 3, 4, 5