Endometrial Cancer Arising in Adenomyosis That Could Not Be Diagnosed by Endometrial Biopsy: A Case Report

Uterine adenomyosis is an estrogen-dependent tumor and one of the most common benign diseases in sexually mature women. The frequency of endometrial cancer associated with adenomyosis has been reported to be 18%–66%. On the other hand, endometrial cancer arising in adenomyosis (EC-AIA) is extremely rare. EC-AIA is now considered a different entity from and has a worse prognosis than endometrial cancer with adenomyosis (EC-A). In the present study, we report a case of endometrial cancer with adenomyosis in which endometrial biopsy failed to provide a definitive diagnosis. A 63-year-old female patient presented with endometrial thickening. Endometrial cytology was positive, and magnetic resonance imaging (MRI) showed small lesions suggestive of endometrial cancer with shallow invasion and adenomyosis. However, an endometrial biopsy showed only metaplasia, and careful follow-up was initiated. Subsequent endometrial cytology showed enlarged and round nuclei, uniform chromatin distribution, no thickening of nuclear margins, and abundant cytoplasm appearing in a sheet-like arrangement, suggesting atypical cells of endometrial glands with metaplasia. Three suspicious positive results and one positive result were observed, but repeated biopsies did not lead to the diagnosis of malignancy. The patient underwent diagnostic hysterectomy 19 months after the initial visit. The postoperative histopathological diagnosis was stage IA endometrial cancer (endometrioid carcinoma G1). This case of endometrial cancer associated with adenomyosis was difficult to diagnose. Our findings demonstrate that EC-AIA should be considered even if no lesions were detected by endometrial biopsy.