T3 Lesion: Colorectal Tumor Invading Through Muscularis Propria into Subserosa
A colorectal tumor that invades through the muscularis propria into the subserosa is classified as a T3 lesion. 1
TNM Classification for Colorectal Cancer
According to the UICC TNM staging (8th edition) classification for colorectal cancer:
- T1: Tumor invades submucosa 1
- T2: Tumor invades muscularis propria 1
- T3: Tumor invades subserosa or into non-peritonealised pericolic or perirectal tissues 1
- T4: Tumor directly invades other organs or structures and/or perforates visceral peritoneum 1
Detailed Explanation of T3 Classification
T3 tumors are defined by:
- Invasion through the muscularis propria 1
- Extension into the subserosa or into non-peritonealised pericolic or perirectal tissues 1
- No perforation of the visceral peritoneum (which would make it T4a) 1
- No direct invasion of adjacent organs or structures (which would make it T4b) 1
Clinical Significance of T3 Classification
T3 tumors have important prognostic implications:
- Prognosis of T3 tumors can be stratified according to the extent of invasion beyond the muscularis propria 1
- A measurement of 5 mm beyond the muscularis propria is considered an accepted cut-off for higher risk in some studies 1, 2
- Recent research suggests subdividing T3 into categories based on depth of invasion: T3a (≤1 mm), T3b (>1-5 mm), T3c (6-15 mm), and T3d (>15 mm) 1, 2
- Five-year distant metastasis rates increase significantly from T3a (5.7%) to T3b,c (17.7%) to T3d (37.2%) 2
Diagnostic Considerations
When evaluating the depth of invasion in colorectal tumors:
- Pelvic MRI is the most accurate test to define locoregional clinical staging for rectal tumors 1
- Endoscopic rectal ultrasound (ERUS) is more valuable for early tumors but less useful in locally advanced rectal cancer 1
- Pathologically, distinguishing between deep muscularis propria invasion and subserosal invasion can be challenging when extensive desmoplasia is present 3
- Immunohistochemical staining using smoothelin and caldesmon can help differentiate muscularis propria from desmoplasia in challenging cases 3
Common Pitfalls in T3 Classification
- Mistaking inflammatory adhesion for tumor invasion: If a tumor appears macroscopically adherent to other structures but no tumor is present in the adhesion microscopically, it should be classified as pT1-3 depending on the anatomical depth of wall invasion 1
- Confusing peritoneal involvement through direct continuity (T4a) with discontinuous peritoneal deposition (M1c) 1
- Failing to recognize that tumor deposits in the subserosa without regional lymph node metastasis should be classified as N1c, not affecting the T category 1