Background: Data registries lack a definitive classification system that distinguishes different locations of colon cancer from one another.

Objective: To establish an international consensus on the definition of primary colon cancer segment locations.

Design: Between December 2022 and June 2023, the Delphi survey study was conducted to seek opinions from relevant international experts and eventually develop a consensus definition of each colon cancer segment.

Setting: Three-round online-based Delphi survey study.

Interventions: The online survey included 17 questions. In the first 2 rounds, participating experts were asked to rank each statement on a scale of 1 (least relevant) to 9 (most relevant). Consensus statements and definitions were revised according to the results for statements obtaining a consensus score of 7 to 9. During the third round and online meeting, definitions and statements that reached a moderate or high consensus (above 4 for more than 70% of participants) were included.

Main outcome measures: The primary goal of our project was focused on precisely localizing the specific segment affected by primary colon cancer rather than identifying surgical treatment or type of resection needed for a particular segment.

Results: The first round included 331 experts; 301 (91%) completed the second round and 295 (98%) completed the final round. Experts strongly supported the use of a "10-cm rule" to describe colon cancer sites at the flexures and anatomical landmarks for other segments. Regarding the definition of rectosigmoid cancer, experts from United States and Europe reached a high consensus that the term rectosigmoid as a colon cancer location must be abolished in contrast to experts from Asia. The description of overlapping segments of cancers achieved a consensus of 64%.

Limitations: Subjective decisions are based on individual expert clinical experience.

Conclusions: This Delphi survey, the first internationally conducted consensus study, achieved a remarkable level of consensus among a panel of global experts. Ambiguity still exists regarding overlapping lesions. See Video Abstract .