AB151. SOH26AB_0140. Lynch syndrome screening in colorectal cancer: audit on adherence to national tumour testing guidelines in Irish hospitals

Background: All newly diagnosed colorectal cancers (CRCs) should undergo universal mismatch repair (MMR) immunohistochemistry (IHC), as high microsatellite instability (MSI-H) or loss of an MMR protein may suggest an inherited predisposition to cancer such as Lynch syndrome. Early detection guides management, surveillance, and genetic counselling to at-risk relatives. When deficient MMR (dMMR) is identified, further testing is required to distinguish sporadic from hereditary CRC. This audit aimed to assess current practice in University Hospital Galway in relation to the tumour testing pathway recommended by the National Cancer Control Programme (NCCP).

Methods: Electronic pathology and multidisciplinary reports were reviewed retrospectively for all patients referred to University Hospital Galway with CRC diagnoses between 2021 and 2024. Data were collected, including MMR IHC status, individual protein expression (MLH1, MSH2, MSH6, and PMS2), MSI-H status; for dMMR cases, molecular testing results (KRAS, NRAS, BRAF, and NTRK), MLH1 methylation testing, and any Lynch syndrome diagnoses.

Results: Among 783 patients, 712 (90.9%) had MMR IHC testing on initial tumour assessment. Twenty patients had dMMR with retained MLH1, nine were referred for genetic services, and of those referred, seven were diagnosed with Lynch Syndrome. The NCCP also recommends patients with MLH1 loss and BRAF wild type to undergo MLH1 methylation testing. Of those meeting these criteria, four patients were identified with unmethylated MLH1, of which three were referred to genetic services, resulting in an overall adherence rate of 60%.

Conclusions: This audit shows that despite consistent initial MMR IHC testing, gaps in reflex testing and referrals remain, potentially delaying early detection of Lynch syndrome, impacting patient outcomes.

Keywords: Colorectal cancer screening (CRC screening); immunohistochemistry (IHC); Lynch syndrome; deficient mismatch repair (dMMR); reflex testing