Assessment of Red Dichromatic Imaging with Indigo Carmine for Identifying Deep Submucosal Invasion in Colorectal Tumors: A Pilot Study

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Background/Objectives: Accurate pT1b diagnosis in colorectal cancer is vital owing to the risk of lymph node metastasis. While Japan NBI (narrow band imaging) Expert Team (JNET) classification is widely applied, accurate diagnosis of type 2B lesions remains challenging, often requiring pit pattern analysis via magnifying chromoendoscopy with crystal violet staining (MCE). However, the clinical application of MCE is limited by potential carcinogenicity and prolonged procedure time. In this study, we aimed to evaluate the diagnostic performance of red dichromatic imaging with indigo carmine (RDI-indigo) in assessing the invasion depth of colorectal tumors. Methods: Ninety images were obtained from 30 colorectal tumor cases using RDI-indigo, NBI, and MCE. Six endoscopists classified images using JNET classification for NBI, and pit pattern classification for RDI-indigo and MCE. JNET type 3 and pit pattern classification V irregular, high grade/V non-structure were correlated with pathological depth ≥pT1b. The primary outcome was the diagnostic accuracy for ≥pT1b. Results: Diagnostic accuracies for ≥pT1b were 85.0% (95% CI: 79.8–90.2%) for RDI-indigo, 79.4% (95% CI: 73.5–85.3%) for NBI, and 82.8% (95% CI: 77.3–88.3%) for MCE. Intraobserver agreement between RDI-indigo and MCE showed fair agreement (Cohen’s kappa = 0.39), while interobserver agreement was moderate for MCE (Fleiss’ kappa = 0.56) and fair for RDI-indigo (Fleiss’ kappa = 0.36). Gwet’s AC1 indicated substantial agreement across all assessments (0.69–0.80). Conclusions: As the first report evaluating RDI-indigo for colorectal tumors, this study suggests that RDI-indigo could serve as a complementary MCE-like tool for the diagnosis of ≥pT1b lesions.

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