What is a T1 low‑risk polyp‑contained colorectal cancer?

T1 Low-Risk Polyp-Contained Colorectal Cancer: Definition and Criteria

A T1 low-risk polyp-contained colorectal cancer is a malignant polyp with submucosal invasion that meets ALL five histological criteria for favorable prognosis: submucosal invasion depth <1 mm, absence of lymphovascular invasion, well or moderately differentiated tumor histology, no tumor budding (or low-grade budding only), and negative resection margins ≥1 mm. 1

Core Definition

T1 colorectal cancer refers to a malignant polyp where cancer has invaded into the submucosa but has not penetrated through the muscularis propria—this is classified as pT1 in the TNM staging system. 1 The term "polyp-contained" indicates the cancer was discovered within a polypoid lesion and was amenable to endoscopic resection. 1

The Five Histological Criteria for Low-Risk Classification

For a T1 cancer to be considered "low-risk" or "favorable prognosis," it must satisfy ALL of the following criteria simultaneously: 1

1. Submucosal invasion depth <1 mm (also termed sm1 or superficial submucosal invasion) 1

2. Absence of lymphovascular invasion (no cancer cells within lymphatic or blood vessels) 1

3. Well-differentiated or moderately differentiated tumor (low histologic grade, not poorly differentiated) 1

4. No tumor budding (or only low-grade budding if present) 1

5. Negative resection margins ≥1 mm (complete excision with adequate margin, no tumor within 1 mm of the resection edge) 1

Clinical Significance and Management Implications

When ALL five favorable criteria are met, the risk of lymph node metastasis drops to approximately 1-2%, making observation after complete endoscopic resection an acceptable curative strategy without additional surgery. 1, 2, 3

The overall lymph node metastasis risk in T1 colorectal cancer is approximately 15% across all cases. 1 However, this risk stratifies dramatically based on histological features. 2, 4, 5, 6

Low-Risk Management

• Patients meeting all five favorable criteria should undergo observation only after complete endoscopic resection, with surveillance colonoscopy, CEA monitoring, abdominal ultrasound, and chest/abdominal CT scans. 1

• Endoscopic follow-up should occur within 3-6 months if there is any uncertainty about margin status, and within 1 year if margins are definitively negative. 1

High-Risk Features Requiring Surgery

If ANY of the following unfavorable features are present, the patient is NOT low-risk and requires surgical resection with regional lymph node dissection: 1

• Submucosal invasion ≥1 mm (or sm2-sm3 levels, particularly ≥1800 μm) 3, 6
• Presence of lymphovascular invasion 1, 2, 6
• Poorly differentiated or mucinous histology 1, 3
• High-grade tumor budding 1, 4, 3
• Positive or indeterminate resection margins (tumor within 1 mm of margin or at electrosurgical edge) 1
• Specimen fragmentation preventing adequate pathological assessment 1

Critical Caveats and Common Pitfalls

The most common error is assuming that endoscopic resection alone is adequate without verifying ALL five favorable criteria. Missing even one unfavorable feature significantly increases lymph node metastasis risk from ~1-2% to 10-15% or higher. 1, 2, 4

Piecemeal resection of suspected malignant polyps should be avoided because specimen fragmentation prevents accurate assessment of invasion depth and margins, automatically placing the patient in the high-risk category requiring surgery. 1, 7

Even with favorable histology, the lymph node metastasis risk is not zero—it remains approximately 1.9% (95% CI: 0.5-4.8%). 2 Patients must be counseled that observation carries a small but real oncologic risk, and comprehensive staging with imaging and tumor markers is mandatory. 1

For sessile (non-pedunculated) polyps with cancer, the threshold for surgery is lower because adverse event rates (recurrence, mortality, hematogenous spread) are significantly higher compared to pedunculated polyps, particularly when margins are positive. 1

Depth of Invasion Measurement Systems

Different classification systems exist for measuring submucosal invasion depth: 4, 3, 5, 6

• Absolute depth measurement: <1 mm is low-risk; ≥1 mm (particularly ≥1.8 mm or sm3) is high-risk 1, 3
• Haggitt classification (for pedunculated polyps): Levels 1-3 are favorable; Level 4 is unfavorable 8
• Kikuchi/sm classification: sm1 (<1000 μm) is favorable; sm2-sm3 (≥1000 μm) is unfavorable 3, 6

The most recent guidelines emphasize the <1 mm cutoff as the critical threshold for low-risk designation. 1