Colonoscopy is a widely used screening method for the detection and removal of growths, known as polyps, within the colon and rectum. Identifying and correctly classifying these polyps is a foundational part of preventing colorectal cancer. Accurate visual diagnosis during the procedure is paramount because it directly influences the patient’s immediate treatment and long-term surveillance schedule. The Japan Narrow Band Imaging Expert Team (JNET) classification is a modern, standardized system developed to help endoscopists make this precise diagnosis in real-time.
The Role of Narrow Band Imaging
The JNET classification relies on an advanced imaging technology called Narrow Band Imaging (NBI). NBI is a form of digital chromoendoscopy that modifies the light source used in the endoscope to enhance the visualization of subtle tissue structures. Conventional colonoscopy uses white light, which contains the entire visible spectrum of colors.
NBI uses an optical filter to isolate only two specific wavelengths of light: blue light (around 415 nanometers) and green light (around 540 nanometers). These wavelengths are chosen because they correspond to the peak absorption characteristics of hemoglobin. The blue light penetrates the most superficial layer, highlighting the fine capillary network on the mucosal surface. The green light penetrates slightly deeper, emphasizing the subepithelial vessels beneath the surface.
By using these two narrow light bands, the blood vessels appear dark against the mucosal background. This provides a high-contrast image of the vascular patterns and surface microstructures, which are often obscured under normal white light. This enhanced visualization allows endoscopists to apply the JNET classification and predict the polyp’s underlying histology.
Why Standardized Classifications Are Essential
Before the introduction of standardized systems like JNET, the interpretation of polyp characteristics during colonoscopy was often subjective and varied between practitioners. Subjective visual assessment can lead to inconsistent diagnoses, potentially resulting in unnecessary surgical procedures or missed lesions that harbor early cancer.
The JNET classification, proposed in 2014, provides a consistent framework for endoscopists to assess polyps using NBI with magnification. This standardization aims to increase consistency and agreement (inter-observer agreement) in diagnoses across different hospitals and countries. By establishing clear criteria, the system enables more accurate “optical diagnosis,” allowing the endoscopist to reliably predict the polyp’s pathology without waiting for a laboratory biopsy. This ability to predict histology in real-time is a significant step toward making immediate treatment decisions.
Defining the JNET Categories
The JNET classification systematically defines four categories by evaluating the appearance of the vessel pattern and the surface pattern of the polyp under magnified NBI. The categories correlate strongly with the predicted pathological finding, dividing lesions into non-neoplastic, low-grade, high-grade, and deeply invasive types.
Type 1
Type 1 lesions are characterized by an invisible vessel pattern, or vessels with a caliber similar to the surrounding normal mucosa, and a surface pattern of regular dark or white spots. This appearance correlates with non-neoplastic polyps, such as hyperplastic polyps or sessile serrated polyps. These polyps have a very low or no cancer risk.
Type 2A
Type 2A lesions display a regular vessel pattern, often appearing meshed or spiral, with a regular surface pattern, such as tubular or papillary structures. This category predicts low-grade intramucosal neoplasia (low-grade adenoma). These polyps are considered neoplastic, meaning they have the potential to become cancerous, but they are not yet high-risk.
Type 2B
Type 2B lesions show a distinct change, featuring a variable caliber and irregular distribution of vessels, often with an irregular or obscure surface pattern. This irregular appearance is associated with high-grade intramucosal neoplasia (high-grade adenoma) or shallow submucosal invasive cancer. The irregularity suggests a more advanced stage of tissue change and a higher potential for malignancy.
Type 3
Type 3 lesions represent the highest risk, characterized by an amorphous, disrupted, or absent surface pattern. They also feature a severely irregular vessel pattern that includes areas of vessel interruption or loose vessel areas. This visual finding strongly predicts deep submucosal invasive cancer. The disorganization of the microvasculature is a key indicator that the cancer has invaded deeper layers of the colon wall.
JNET and Immediate Treatment Pathways
The optical diagnosis provided by the JNET classification directly dictates the immediate management strategy during the colonoscopy. This is known as the “resect and discard” or “diagnose and leave” approach, which streamlines patient care.
For a JNET Type 1 diagnosis, the endoscopist may elect to simply leave the polyp in place or remove it using a simple technique, as the risk is minimal. JNET Type 2A lesions (low-grade adenomas) typically lead to immediate endoscopic removal. These procedures, often standard polypectomy or endoscopic mucosal resection (EMR), can be performed safely during the same session.
A classification of JNET Type 2B indicates a higher probability of high-grade dysplasia or shallow cancer, requiring a cautious approach. These lesions are usually removed using advanced endoscopic techniques, such as endoscopic submucosal dissection (ESD), to ensure the entire lesion is removed in one piece (en bloc resection).
If a JNET Type 3 lesion is identified, the endoscopist typically halts any attempt at endoscopic removal. The finding of deep submucosal invasive cancer necessitates a referral for surgical resection, as the cancer has likely invaded too deeply for safe endoscopic removal.