The risk factors for metastasis in T1 colon cancer and the criteria for additional resection following endoscopic treatment are important topics. In recent years, several reports have emerged suggesting that among risk factors, the depth of invasion shows a weak correlation with metastasis. Furthermore, there are reports indicating differences in recurrence risk and prognosis between the rectum and the colon. Therefore, we aimed to clarify the impact of tumor location on metastasis risk using a multicenter prospective registry database of colorectal T1 cancers without considering invasion depth.The risk factors for metastasis in T1 colon cancer and the criteria for additional resection following endoscopic treatment are important topics. In recent years, several reports have emerged suggesting that among risk factors, the depth of invasion shows a weak correlation with metastasis. Furthermore, there are reports indicating differences in recurrence risk and prognosis between the rectum and the colon. Therefore, we aimed to clarify the impact of tumor location on metastasis risk using a multicenter prospective registry database of colorectal T1 cancers without considering invasion depth.

Patients aged 20 years or older diagnosed with colorectal pT1 cancer and resected endoscopically or surgically at 48 Japanese institutions were prospectively enrolled after obtaining consent for cohort study participation. Exclusion criteria were inflammatory bowel disease associated colorectal cancer, familial adenomatous polyposis, coexisting or past history of invasive colorectal cancer, and active cancer in other organs. The rectum was defined as the upper rectum (Ra) and the lower rectum (Rb), and the colon as all other locations. The metastasis high-risk group was defined as cases not meeting all of the following pathological criteria without considering invasion depth: 1) no lymphatic invasion (Ly0), 2) no venous invasion (V0), 3) tumor budding grade1 (BD1), and 4) well to moderate differentiated tubular adenocarcinoma (tub1 or tub2). This study compared the proportion of high-risk groups for metastasis in the rectum and colon using data at the time of registration.

Between May 2017 and January 2021, 1527 patients were registered, and 1479 patients were included after excluding ineligible cases. The final analyzable patients were 351 rectal cases and 1128 colon cases. Their respective backgrounds were: male 67%/58%, median age 68/70 years old, macroscopic type including depressed type 27%/31%, median tumor size 20/18 mm, and initial endoscopic treatment (including surgical local resection) 78.0%/68.1%. The proportion of high-risk groups for metastasis was 55.0% in the rectal group and 40.3% in the colonic group (P<.001), with an odds ratio of 1.18 (95% confidence interval 1.42-2.30). Furthermore, when limited to cases with a lesion diameter <10 mm, the rectal group was 78.6% and the colonic group was 38.1%, with an odds ratio of 5.95 (95% confidence interval 1.56-22.72).

When evaluating the risk of metastasis in T1 colorectal cancers without considering invasion depth, the rectum was found to be a significantly higher risk than the colon. This was particularly pronounced in cases with a diameter of less than 10 mm. A careful pre-treatment evaluation is necessary, and caution should be exercised against performing cold snare polypectomy too readily.