Background: Right-sided serrated polyps are precursors to sporadic microsatellite unstable colon cancers via the methylator pathway and have a high rate of synchronous and metachronous lesions. Serrated polyps also occur in Lynch syndrome, where right-sided microsatellite unstable cancers arise from germline mutations in mismatch repair genes.

Objective: The aim of this study was to compare serrated neoplasia in patients with sporadic and hereditary microsatellite unstable colon cancer and to examine the effect of right colectomy on the risk of metachronous polyps and cancers.

Design: This is a retrospective, descriptive, cohort study from database and chart review.

Setting: This study was conducted at a tertiary care hospital with a center for hereditary colorectal cancer.

Patients: Patients who had colon cancers with a high degree of microsatellite instability, methylator cancers, and Lynch syndrome cancers, were included.

Interventions: Interventions included colectomy, surveillance colonoscopy, and polypectomy.

Main outcome measures: The primary outcomes measured were the incidence and location of metachronous polyps and cancers.

Results: Eighty-five patients were included: 47 with methylator cancers and 38 with Lynch syndrome. Median ages at surgery were 75 years (range, 41-90) and 48 years (range, 27-77), p < 0.0001. Forty-six (98%) patients with methylator cancers and 17 (45%) patients with Lynch syndrome underwent a right colectomy, p < 0.0001. Metachronous cancers occurred in 19/60 (32%) of patients with Lynch syndrome and no patients with methylator cancers, p < 0.0001. Thirty-four patients with methylator cancers had colonoscopic follow-up, with a median of 2 colonoscopies per patient over a 32-month follow-up (range, 1-136). Sixty-three percent of patients with Lynch syndrome had colonoscopic follow-up, median of 4 colonoscopies per patient over 102 months (range, 1-462), p < 0.0001. Four (9%) patients with methylator cancers each had 1 metachronous serrated polyp, compared with 10/37 (27%) patients with Lynch syndrome (p = 0.049), whose median number of polyps was 2 (range, 1-8). Characteristics of other associated polyps were similar between cohorts.

Limitations: This study is somewhat limited by potential inherent bias from its retrospective design. Also, a high number of deaths in the CIMP+ cohort could have contributed to the low number of serrated polyps detected on colonoscopy surveillance, but given current understanding of serrated polyp growth, this may truly represent the left colon's tendency not to develop serrated polyps.

Conclusions: Cancers with a high degree of microsatellite instability arise through 2 different molecular mechanisms. Metachronous serrated neoplasia, benign and malignant, following right colectomy in patients with the CpG-island methylator phenotype of colorectal cancer is uncommon. However, the colons of patients with Lynch syndrome are at high risk after segmental colectomy.