Cross-Modal Relational Distillation for White-Light Endoscopic Gastrointestinal Lesion Classification

White-light imaging (WLI) is widely used for colorectal polyp assessment, whereas narrow-band imaging (NBI) reveals vascular and mucosal details but requires dedicated imaging equipment. Existing cross-modal distillation methods use verified WLI–NBI pairs for direct transfer or unpaired images for group-level supervision, leaving the relational information accessible through verified pairs underexplored. We propose Paired-anchor Cross-modal Transfer (PaCT), which uses verified pairs as shared identity anchors to transfer NBI relations to unpaired WLI images. PaCT builds an NBI teacher graph from paired and additional unpaired NBI images. For each unpaired WLI query, its affinities to WLI anchors are mapped through the verified identities to obtain a query-specific relational target from the NBI graph, without assuming a match to any unpaired NBI image. Under a common three-seed, five-fold evaluation protocol, PaCT improves mean accuracy, F1-score, and AUC over the best comparator for each metric by 2.48, 2.64, and 2.71 percentage points on PICCOLO, and by 1.93, 2.13, and 2.82 percentage points on PolypSet, respectively. PaCT thus extends the utility of verified pairs beyond paired examples while requiring only WLI at inference. An anonymous code repository is available at https://anonymous.4open.science/r/PaCT-iclr2027-code/