Introduction Endometrial cancer (EC) is the most common gynaecological malignancy in New Zealand. Pacific women have the highest incidence, which is rising in those under 50 years of age. The introduction of immunohistochemistry for EC has important implications for identification of potential Lynch syndrome (LS). Universal testing of EC tumours for a mutation in one of the DNA mismatch repair genes (MMR) was introduced to New Zealand in 2017. The objective of this study was to investigate the rate of MMR deficient and proficient tumours within our population, and whether these rates vary according to ethnicity.

Methods This is a retrospective population-based cohort study of all cases of EC diagnosed between 1st January 2017 until 31 December 2018 within the Auckland region. Incidence of MMR deficient and proficient tumours was assessed for each ethnicity and compared.

Results 409 patients were diagnosed with EC, 81.6% (n=334/409) underwent MMR IHC testing. There were 266 pMMR (79.6%) and 68 dMMR (20.4%) EC tumours. 26.1% of EC in European patients were dMMR, compared with 10% in Māori (p=0.06, RR 0.4 (0.1 – 1.2)), and 11.4% in Pacific (p=0.004 RR 0.5 (0.3 – 0.9)), and 28.3% in Asian (ns). 8 patients (2.3%) were diagnosed with Lynch Syndrome: 4/8 (50%) European, 2/8 (25%) Asian, 1/8 (12.5%) Indian, 1/8 (12.5%) Middle Eastern.

Conclusion/Implications Despite having an increased incidence of EC in New Zealand, Māori and Pacific people have significantly lower rates of MMR deficient tumours than the European population. None of the Pacific or Māori patients had Lynch syndrome.