Background: Microsatellite instability-high is a known biomarker for anti-PD-1/PD-L1 immune checkpoint therapy. It is also a known tumour feature of Lynch syndrome, detected most frequently in endometrial cancer. However, it remains unclear how microsatellite instability testing is carried out in the clinical field. Methods: Ninety-nine patients with gynaecological malignant tumours who underwent microsatellite instability testing as a companion diagnosis for pembrolizumab and 16 patients who previously underwent microsatellite instability testing as a screening for Lynch syndrome were recruited. Clinical information, microsatellite instability status, outcomes, genetic assessments and information about cancer tissue were retrospectively analysed. Results: Ninety-nine patients had 101 gynaecologic malignant tumours including 26 endometrial, 38 ovarian and 28 cervical cancers, 9 with other tumours including 2 synchronous endometrial and ovarian cancers. All tissue samples were successfully tested, even though some were ≥10-year-old samples. Three cases (3.0%, 3/99) showed microsatellite instability-high; all cases were endometrial cancers with one case of synchronous endometrial and ovarian cancer [11.5% (3/26) in endometrial cancer, 2.6% (1/38) in ovarian cancer], and there was no microsatellite instability-high in cervical and other cancers. One of the endometrial cancer patients received pembrolizumab treatment, but finally died of cancer. Two other cases underwent genetic testing; both were diagnosed as Lynch syndrome. Six cases (37.5%) showed microsatellite instability-high in screening for Lynch syndrome. Conclusions: Microsatellite instability-high was less commonly detected as a companion diagnosis for pembrolizumab in unselected gynaecologic patients. Genetic counselling should be always provided along with treatment selection.
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