A 55-year-old man undergoes resection of a colonic mass. Histopathology shows a well-differentiated adenocarcinoma invading through the muscularis propria into the pericolorectal fat, with 3 of 18 regional lymph nodes positive for metastatic carcinoma. No distant metastases are identified. According to TNM staging, what is the stage of this tumor?

• A Stage I (T2 N0 M0)
• B Stage II (T3 N0 M0)
• C Stage IV (T3 N1 M1)
• D Stage III (T3 N1 M0) ✓

Explanation

Invasion through the muscularis propria into pericolorectal fat is classified as T3 in the AJCC colorectal TNM system. Involvement of 1–3 regional lymph nodes is N1. With no distant metastases (M0), this combination yields Stage III disease. Stage II requires N0 (no nodal involvement), and Stage IV requires M1 (distant metastasis).

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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