Introduction A human sporadic colorectal adenoma may influence the formation of another adenoma even after its removal, with up to 60% of such patients developing metachronous adenomas following adenoma excision. However data regarding the occurrence site of a metachronous colorectal adenoma relative to the index adenoma is scarce. Therefore, we investigated whether a metachronous colorectal adenoma was more likely to occur in the same, proximal or distal segment as the index adenoma.

Methods A prospectively–maintained database was interrogated to identify all colonoscopies performed over a ten-year period at a single university teaching hospital. All patients undergoing polypectomy were cross-referenced against a histological database to confirm adenoma status. Patients with synchronous adenomas were excluded. The site of adenoma removal at index and subsequent colonoscopy was recorded and categorised into three groups (proximal, distal or same segment).

Results 15,121 colonoscopies and 4759 polyp events were recorded. 361 patients (235 male, 126 female, median age [range] 66 [32–91] years) developed a single metachronous adenoma at follow-up colonoscopy. Metachronous adenomas were more likely to develop in a different bowel segment (61%, 95% c.i. 56% > 66%) to that of the index adenoma (39%, 95% c.i. 34% > 44%; P < 0.01 one way Chi-squared test). Metachronous adenomas were more likely to occur at a site proximal (41%, 95% c.i. 36% > 46%) to the index adenoma than either the same segment (39%, 95% c.i. 34% > 44%) or a more distal segment (20%, 95% c.i. 16% > 24%; P < 0.01 one way Chi-squared test). Proximally-sited metachronous adenomas were more likely to occur in a segment further away (mean [SD] segments travelled 3.5 [2.3]) from the index adenoma than distally-sited metachronous adenomas (2.6 [1.8] segments travelled; P < 0.01 Kruskal-Wallis one way ANOVA).

Conclusion A metachronous human sporadic colorectal adenoma is more likely to be found in a section of the colorectum proximal to that of the index adenoma. Travel in segments may be significant. Taken together, the data suggest research effort into underlying mechanisms of metachronous adenoma is needed.

Disclosure of Interest None Declared.