Clinical implications of microsatellite instability in mucinous colorectal cancer.
657 Background: Colorectal Cancer(CRC) is becoming increasingly recognised as a heterogeneous tumor type. Mucinous histological subtype is identified in 10-15% of CRCs, most commonly those with microsatellite instability (MSI), and has traditionally been associated with unfavorable outcomes and poor response to chemotherapy. In contrast, MSI is associated with relatively favourable pathological features and better outcomes, compared with CRCs with microsatellite stability (MSS), such that under the 2010 WHO classification, MSI mucinous CRC is considered low grade, while MSS mucinous CRC is classified as high grade. The aim of this study is to establish the significance of microsatellite stability status in non-metastatic mucinous colorectal cancer. Methods: Between 2010 and 2017, 69 patients with stage II or stage III mucinous colorectal cancer were identified. Microsatellite status was tested in all patients (MSS or MSI), and histological and clinical data, as well as recurrence rates, were assessed in both groups. MSI status was established using polymerase chain reaction(PCR) technique. Results: Sixty-nine patients with mucinous CRC were identified. The median age for the entire group was 73 years (range 32-87), no difference in gender was identified. 63%(n=43) and 37%(n=26) were stage II and stage III respectively at diagnosis. The majority of mucinous CRCs were right-sided (72%). 33% (n=23) were identified as microsatellite unstable (MSI). MSI status was associated with right sided tumours (78% right-sided vs 22% left-sided, p<0.05), older age at diagnosis (mean 76 years vs 68 years, p=0.01), and lower TNM staging at diagnosis (83% vs 52% diagnosed stage 2, p=0.007) compared with the MSS group. A lower disease recurrence rate was identified in the MSI group (4.3% vs 13% in MSS group) at median follow-up time of 33 months (range 8-93 months). Conclusions: In patients with mucinous colorectal cancer, MSI status is a useful marker of favourable histological and clinical features, and is associated with better outcomes. Our study supports the current 2010 WHO classification, and highlights the clinical and prognostic significance of MSI status in this patient cohort.