What is the classification of polyps, specifically colorectal polyps, based on their histological characteristics?

Classification of Colorectal Polyps

Colorectal polyps are classified by three complementary systems: histological type (neoplastic vs. non-neoplastic), morphological appearance using the Paris classification, and optical characteristics using the NICE classification, with histological classification being the gold standard for determining malignant potential and guiding management. 1, 2, 3

Histological Classification

Colorectal polyps are fundamentally divided into neoplastic and non-neoplastic categories based on their malignant potential 3:

Neoplastic Polyps (Adenomatous)

• Tubular adenomas: Account for 90.6% of small neoplastic polyps and represent the most common adenomatous subtype 4
• Tubulovillous adenomas: Mixed architecture with both tubular and villous components
• Villous adenomas: Higher malignant potential than tubular adenomas
• Serrated adenomas: Including traditional serrated adenomas 5

Non-Neoplastic Polyps

• Hyperplastic polyps: Comprise 23.9% of small colorectal polyps and lack malignant potential 4
• Hamartomatous polyps: Including juvenile polyps and Peutz-Jeghers polyps 5
• Inflammatory polyps: Associated with inflammatory bowel disease 3

Critical distinction: Approximately 60.4% of small colorectal polyps (≤8mm) are neoplastic, meaning the majority harbor malignant potential and require removal 4. The adenoma-to-carcinoma sequence establishes that most colorectal carcinomas evolve from adenomatous polyps 3.

Morphological Classification: Paris System

The Paris classification is the standard nomenclature for describing surface morphology and should be documented in all procedure reports 1:

Type 0-I: Polypoid Lesions

• 0-Ip (Pedunculated): Polyps with a distinct stalk; easier to resect endoscopically 1, 3
• 0-Is (Sessile): Flat-based polyps arising directly from the mucosa without a stalk 1, 3

Type 0-II: Non-Polypoid Lesions

• 0-IIa (Superficially elevated): Flat elevated lesions with 0.7%-2.4% submucosal invasion risk 1
• 0-IIb (Flat): Completely flat lesions at mucosal level 1
• 0-IIc (Slightly depressed): Highest malignancy risk with 27%-35.9% submucosal invasion rate; requires careful evaluation 1

Type 0-III: Excavated Lesions

• Ulcerated or deeply depressed lesions, typically representing advanced cancer 1

Documentation requirements: Photo documentation of all lesions ≥10mm before removal is strongly recommended, with documentation of location, size in millimeters, and Paris morphology mandatory 1.

Laterally Spreading Tumor (LST) Classification

For non-pedunculated adenomatous lesions (Paris 0-II and 0-Is) ≥10mm, additional LST classification is required 1:

LST-Granular (LST-G)

• Homogeneous type: Even-sized nodules with lowest submucosal invasion risk (<2%) 1
• Mixed nodular type: Mixed-sized nodules with higher invasion risk 1

LST-Non-Granular (LST-NG)

• Flat elevated type: Lower invasion risk 1
• Pseudodepressed type: Highest invasion risk (27.8% for 10-19mm; 41.4% for 20-29mm) 1

Optical Classification: NICE System

The NICE classification uses narrow-band imaging to predict histology in real-time 2:

Type 1 (Serrated Class)

• Pale color, invisible vessels, regular surface with dark/white spots 2
• Predicts hyperplastic or sessile serrated polyps 2

Type 2 (Conventional Adenoma)

• Brown color, regular brown vessels surrounding white structures, tubular/branched surface pattern 2
• Predicts adenomatous histology 2

Type 3 (Deep Submucosal Invasive Cancer)

• Brown to dark brown color, disrupted/missing vessels, amorphous surface pattern 2
• Highly specific (though not sensitive) for deep submucosal invasive cancer 2

Important caveat: Despite high accuracy in reference centers, routine screening colonoscopy achieves only 76.6% accuracy for endoscopic polyp classification, with image-based surveillance recommendations correct in only 69.5% of cases 6. Therefore, histopathological assessment remains mandatory and cannot be replaced by optical diagnosis alone 6.

Polyposis Syndrome Classification

When ≥10 cumulative polyps are present, consider hereditary polyposis syndromes 5:

Adenomatous Polyposis

• Familial Adenomatous Polyposis (FAP): ≥10 cumulative adenomatous polyps; refer for genetic assessment 5, 7
• MUTYH-Associated Polyposis (MAP): Similar polyp burden to FAP 5

Hamartomatous Polyposis

• Juvenile Polyposis Syndrome (JPS): 3-5 cumulative juvenile polyps or multiple throughout GI tract 5, 7
• Peutz-Jeghers Syndrome (PJS): ≥2 PJ polyps or ≥1 with mucocutaneous hyperpigmentation 5, 7
• PTEN Hamartoma Tumor Syndrome: Gastric juvenile polyposis-type polyps with other features 7

Serrated Polyposis

• Serrated Polyposis Syndrome (SPS): ≥5 serrated polyps proximal to sigmoid (two >1cm) OR >20 serrated polyps anywhere 5

Mixed Polyposis

• Hereditary Mixed Polyposis Syndrome (HMPS): ≥10 cumulative polyps with >1 histology 5, 7

Malignant Polyp Risk Stratification

For polyps containing invasive carcinoma, histologic features determine risk of lymph node metastasis 5, 8:

High-Risk (Unfavorable) Features

Any single unfavorable feature mandates surgical resection 5:

• Poor tumor differentiation 5, 8
• Lymphovascular invasion (strongest predictor with RR 5.2) 5, 8
• Tumor budding (RR 5.1 for lymph node metastasis) 5
• Positive margins: <1mm for pedunculated polyps; tumor within cauterized margin for sessile polyps 5
• Deep submucosal invasion: >1mm depth (RR 5.2) 5

Low-Risk (Favorable) Features

Polypectomy is considered curative when all of the following are present 5, 8:

• Well or moderately differentiated tumor
• No lymphovascular invasion
• Negative margins (≥1mm for pedunculated)
• Submucosal invasion ≤1mm

Critical pitfall: Pseudoinvasion (displacement of benign glandular epithelium into submucosa) can mimic invasive carcinoma, particularly in large pedunculated sigmoid polyps with hemorrhage and hemosiderin deposition; level sections and second opinions are essential 5. Pseudoinvasion has no malignant potential and should be treated as adenoma 5.