Primary Tumor (T)
TX – primary tumor cannot be assessed
T0 – no evidence of primary tumor
Tis – carcinoma in situ: intraepithelial or invasion of lamina propria
T1 – tumor invades submucosa
T2 – tumor invades muscularis propria
T3 – tumor invades through muscularis propria into subserosa or into nonperitonealized pericolic or perirectal tissues
T4 – tumor directly invades other organs or structures and/or perforates visceral peritoneum

Regional Lymph Nodes (N)
NX – regional lymph nodes cannot be assessed
N0 – no regional lymph node metastasis
N1 – metastasis in one to three regional lymph nodes
N2 – metastasis in four or more regional lymph nodes

Distant Metastases (M)
MX – distant metastasis cannot be assessed
M0 – no distant metastasis
M1 – distant metastasis

Dukes’ Classification (Astler-Coller modification)

Stage A:tumors invade through the muscularis mucosae into the submucosa but do not reach the muscularis propria

Stage B1: tumors invade into the muscularis propria
Stage B2: tumors completely penetrate the smooth muscle layer into the serosa

Stage C: tumors encompass any degree of invasion but are defined by regional lymph node involvement
Stage C1: tumors invade the muscularis propria with fewer than four positive nodes
Stage C2:tumors completely penetrate the smooth muscle layer into the serosa with four or more involved nodes

Stage D: lesions with distant metastases

Carcinoma in situ:
(may be referred to as high grade dysplasia) – intramucosal carcinoma that does not penetrate the muscularis mucosae