Which TNM stage of colorectal cancer is defined by invasion through the muscularis propria into the subserosa or non-peritonealized pericolic tissues, regardless of lymph node involvement?

Explanation

## TNM Staging of Colorectal Cancer **Key Point:** The T3 stage is defined by tumour invasion through the muscularis propria into the subserosa or non-peritonealized pericolic/perirectal tissues. When N0 (no regional lymph node metastasis) and M0 (no distant metastasis), this corresponds to Stage IIA. ### T-Stage Definitions | T Stage | Definition | |---------|------------| | T1 | Invasion into submucosa | | T2 | Invasion into muscularis propria | | T3 | Invasion through muscularis propria into subserosa or non-peritonealized tissues | | T4a | Invasion through visceral peritoneum | | T4b | Direct invasion into adjacent structures | ### Stage IIA Characteristics **High-Yield:** Stage IIA (T3N0M0) is a locally advanced tumour without nodal involvement. These patients have a 5-year survival rate of approximately 70–75% and are candidates for adjuvant chemotherapy in selected cases (e.g., high-risk features such as poor differentiation, lymphovascular invasion, or inadequate lymph node sampling). **Clinical Pearl:** The distinction between T3 and T4a is critical: T3 tumours do not breach the peritoneal surface, whereas T4a tumours penetrate the visceral peritoneum. This affects surgical planning and prognosis. **Mnemonic:** **TNMM** — **T**umour depth, **N**ode involvement, **M**etastasis, **M**akes the stage. T3N0M0 = Stage IIA.