Frequency of mismatch repair deficiency for primary endometrial neuroendocrine carcinoma and its therapeutic outcome in Japan

OBJECTIVE: To investigate the frequency of mismatch repair (MMR) deficiency in primary endometrial neuroendocrine carcinoma (PENEC) and analyze its therapeutic outcomes in Japan. MATERIALS AND METHODS: A multicenter retrospective study of 17 cases of PENEC diagnosed between 2011 and 2023 was conducted. MMR status was evaluated by immunohistochemistry, and clinical outcomes were assessed. RESULTS: Among the 17 cases, 10 (58.8%) were diagnosed at advanced stages (stage III and Ⅳ). The median overall survival was 16 months, indicating extremely poor prognosis. MMR deficiency was observed in 41% of cases, a rate higher than that reported for other endometrial cancer subtypes. CONCLUSION: PENEC demonstrates aggressive behavior and poor prognosis compared to other histological subtypes of endometrial cancer. The higher rate of MMR deficiency suggests that immune checkpoint inhibitors may serve as key components of initial treatment strategies, potentially contributing to improved outcomes for this rare and aggressive malignancy.