Paris Classification for Gastrointestinal Polyps
The Paris classification is an endoscopic morphological classification system for superficial gastrointestinal lesions that categorizes lesions into three major types: polypoid (Type 0-I), non-polypoid (Type 0-II), and excavated (Type 0-III), with specific subtypes that predict risk of submucosal invasion and guide resection strategy. 1
Definition and Purpose
The Paris classification, proposed in 2002, defines superficial lesions as those where endoscopic appearance suggests depth of penetration limited to the submucosa without infiltration of the muscularis propria 1. This system provides standardized nomenclature for describing colorectal lesion morphology and helps stratify risk of advanced pathology 2.
Classification System Structure
Type 0-I: Polypoid Lesions
- 0-Ip (Pedunculated): Lesions with a distinct stalk 1, 2
- 0-Is (Sessile): Lesions without a stalk, protruding above the mucosal surface 1, 2
Type 0-II: Non-Polypoid Lesions
- 0-IIa (Slightly elevated): Lesions elevated less than 2.5 mm above the mucosal surface 2
- 0-IIb (Flat): Lesions with no elevation or depression 2
- 0-IIc (Slightly depressed): Lesions with slight depression below the mucosal surface 1, 2
Type 0-III: Excavated Lesions
Rarely seen in the colon 1
Clinical Significance and Risk Stratification
Submucosal Invasion Risk by Morphology
Depressed (0-IIc) lesions carry the highest risk of submucosal invasion, with overall risk of 27-35.9% compared to 0.7-2.4% in flat (0-IIa) lesions 2. More than 40% of small (6-10 mm) depressed lesions contain submucosal invasive cancer, and virtually all large (>20 mm) depressed lesions have submucosal invasion 2.
Sessile (0-Is) lesions demonstrate increased risk compared to flat lesions, with odds ratio of 2.73 (95% CI 1.64-4.55) for submucosal invasive cancer in lesions ≥20 mm 1.
Laterally Spreading Tumors (LST) Classification
For non-polypoid lesions ≥10 mm, the Paris classification should be supplemented with LST subtyping 2:
LST-Granular (LST-G)
- LST-G homogeneous (even-sized nodules): Lowest risk of submucosal invasion (<2% regardless of size) 1
- LST-G mixed nodules: Higher risk (7.1% for <20 mm; 38% for >20 mm), with invasion typically under the largest nodule 1
LST-Non-Granular (LST-NG)
- LST-NG flat elevated: Moderate risk (6.4% for 10-19 mm; 10.4% for 20-29 mm) 1
- LST-NG pseudodepressed: Highest risk (27.8% for 10-19 mm; 41.4% for 20-29 mm) with typically multifocal invasion points 1
Practical Application and Limitations
Documentation Requirements
The US Multi-Society Task Force recommends documenting location, size in millimeters, and Paris morphology in all colonoscopy reports 2. Photo documentation is strongly recommended for all lesions ≥10 mm before removal 2.
Interobserver Agreement Concerns
A critical caveat is that interobserver agreement of the Paris classification among expert endoscopists is only modest 1. Recent systematic review data confirms moderate agreement for colorectal neoplasms (κ = 0.42 to 0.54) 3. A 2025 video-based study demonstrated low overall inter-observer agreement (Light's Kappa 0.19) among surgeons, gastroenterologists, trainees, and experts 4. Notably, endoscopists trained in Asia, where greater emphasis is placed on lesion characterization, showed higher agreement (κ = 0.50) compared to Western-trained endoscopists (κ = 0.26) 4.
Integration with Advanced Imaging
The Paris classification should be combined with advanced imaging techniques (narrow-band imaging, chromoendoscopy) to assess surface patterns and predict histology 2. The NICE classification and pit pattern analysis complement morphological assessment for determining resection strategy 1.
Treatment Implications
Morphology directly guides resection approach: En bloc resection should be considered for lesions with 0-IIc components, LST-NG morphology, or LST-G with dominant nodules to optimize histologic assessment 1. All pedunculated polyps should be resected en bloc regardless of size 1.