Clinical utility of diagnosing Lynch syndrome in gynecologic oncology: a joint statement from four Japanese academic societies.

Lynch syndrome (LS) is an autosomal dominant cancer predisposition syndrome, associated with substantially increased risks of colorectal and gynecologic malignancies. Endometrial cancer may serve as a sentinel cancer for LS, placing gynecologists and gynecologic oncologists in a key position for early recognition and long-term management. This joint statement was developed through a multidisciplinary process involving the Japan Society of Gynecologic Oncology, the Japan Society of Clinical Oncology, the Japanese Society of Hereditary Tumors, and the Japanese Society for Cancer of the Colon and Rectum. It aims to clarify the clinical utility of diagnosing LS in gynecologic oncology within the Japanese healthcare system. We outline a tumor-first paradigm in which universal mismatch repair immunohistochemistry and/or microsatellite instability testing for endometrial cancer facilitates LS detection and provides actionable biomarkers for systemic therapy. This statement summarizes the clinical impact of LS diagnosis across gynecologic oncology. We highlight patterns of synchronous and metachronous malignancies and the prevalence of LS in ovarian cancer, particularly in endometrioid and clear-cell subtypes. We discuss surgical implications, including risk-reducing hysterectomy with bilateral salpingo-oophorectomy, consideration of concomitant gynecologic risk-reducing surgery during colorectal cancer resection, and individualized ovarian preservation in selected early-stage endometrial cancer or atypical endometrial hyperplasia. We also review the treatment relevance of deficient mismatch repair/microsatellite instability-high status for immune checkpoint inhibitors and emerging fertility-preserving approaches, and address surveillance, cascade testing for relatives, implementation barriers, and the need for multidisciplinary pathways and healthcare system-level support to ensure equitable access to genetic counseling and testing.