Inter and intra-observer agreement for the LST Classification in Large (>2cm) Colorectal Laterally Spreading Tumours

Background: Morphological assessment of large (≥20 mm) colorectal laterally spreading tumors (LSTs) is central to predicting submucosal invasion risk, procedural difficulty, and optimal resection strategy. The LST classification is widely used; however, its diagnostic reliability has never been formally evaluated. Patients and Methods: We performed a prospective multicenter video-based study. 24 blinded expert endoscopists from North America, Europe, Asia, and Oceania independently classified video recorded LSTs twice, in randomized order, into 5 prespecified LST subtypes. Inter- and intra-observer agreement were calculated using Light’s kappa. Secondary analyses evaluated agreement by LST size category, endoscopist sex, geographic training region, and a simplified 3-category granular vs non-granular classification. Results: 46 LSTs were evaluated, yielding 1104 classifications. Inter-observer agreement for the primary outcome was poor in both the first (κ=0.37) and second assessment rounds (κ=0.36). Agreement varied modestly across LST size groups and endoscopist characteristics, with moderate agreement in Europe and Oceania and fair agreement in North America and Asia. Simplifying morphology into 3 categories improved inter-observer agreement to the moderate range (κ≈0.48). Intra-observer agreement ranged widely (κ=0.28–0.94), with the highest reproducibility among endoscopists trained in Europe and Oceania. Intra-observer agreement was generally higher for smaller lesions (<40 mm) and improved when using the 3-category model. Conclusions: Among international experts, inter-observer agreement for the LST classification is low, with considerable inter- and intra-observer variability. These findings highlight the need for improved standardization and suggest caution when using detailed LST subclassification for clinical decision-making or as an endpoint in research.