Aim: Tamoxifen (TAM) is widely used in adjuvant endocrine therapy for invasive breast cancer as a selective estrogen modulator, but this treatment has a risk of developing endometrial malignancy. However, hysteroscopic findings during or after TAM treatment are unclear. The aim of this study is to examine the association between hysteroscopic patterns and malignant histological findings during or after treatment with TAM. Methods: The subjects were patients who received TAM after surgery for breast cancer and underwent hysteroscopy at our institution from January 2016 to December 2019. Clinicopathological factors and hysteroscopic findings were collected from medical records and investigated retrospectively. Histologically, atypical endometrial hyperplasia, endometrial cancer, and carcinosarcoma were classified as malignant diseases. Results: A total of 26 patients were eligible for the study. Hysteroscopic findings included an irregular surface of the endometrium (n = 3, 11.5%), atypical vessels (n = 10, 38.5%), papillary structure (n = 3, 11.5%), and polypoid structure (n = 18, 69.2%). Histological examination revealed malignancy in six patients (23.0%). The percentage of atypical vessels in patients with malignancies was significantly higher than that in patients with a normal endometrium or benign lesion (100% vs. 20%, p = 0.0009). The sensitivity and specificity of atypical vessels in hysteroscopy for diagnosis of malignant diseases were 100% and 80%, respectively. Conclusions: Hysteroscopic findings of atypical vessels may be useful for prediction of malignant diseases in patients treated with TAM.
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