A case of uterine corpus large cell neuroendocrine carcinoma showing prominent myometrial invasion without any macroscopically clear tumor formation.

Large cell neuroendocrine carcinoma (LCNEC) arising in the uterine corpus is a very rare. Here, we report our experience with a primary LCNEC in the uterine corpus that showed prominent myometrial invasion without exhibiting any macroscopically distinct tumor formation in the uterine cavity. The patient was a 54-year-old woman. She had a past medical history of right breast cancer and was referred to our department with irregular genital bleeding, elevated serum carcinoembryonic antigen in periodic medical examinations and computed tomography (CT) findings of uterine cavity dilation. Endometrial biopsy suggested a poorly differentiated tumor. Although magnetic resonance imaging (MRI) showed hematometra-like findings in the uterine cavity, it did not indicate any clear endometrial lesion. The myometrium was unequally thickened, and the entire muscle layer showed a high signal intensity on diffusion-weighted images. Fluorine-18-deoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) showed strong FDG accumulation in the whole uterus, and on the bottom of the uterus, there was a ring-shaped accumulation mainly in the muscle layer. The postoperative resected specimen did not show any tumor formation in the uterine cavity, whereas the myometrium was hard and thickened, and colored white overall. Histopathological examination revealed prominent myometrial invasion in most layers, cervical stromal invasion and pelvic lymph node metastasis. The diagnosis was a LCNEC of the uterine corpus, at FIGO stage IIIC1 and pT2N1M0. With these patients, we found that functional metabolic images, such as MRI diffusion-weighted images and FDG-PET/CT, were useful in identifying the lesion. Preoperatively, when a poorly differentiated tumor is estimated and characteristic myometrial invasion is suspected, the possibility of LCNEC should be considered.