Case Report: Intramyometrial uterine large-cell neuroendocrine carcinoma mimicking adenomyosis and leiomyomas with rapid fatal relapse.

Background: Large-cell neuroendocrine carcinoma (LCNEC) of the female genital tract is rare and highly aggressive. When the dominant tumor burden is intramyometrial, preoperative imaging may mimic benign myometrial disease and delay oncologic planning. Case presentation: A 59-year-old postmenopausal woman presented with progressive lower abdominal pain for over 1 month. Laboratory testing showed elevated carcinoembryonic antigen (CEA) and cancer antigen 125 (CA125), inflammatory changes, anemia, and renal dysfunction. Preoperative transabdominal/transvaginal ultrasonography, whole-abdomen Doppler ultrasonography, chest computed tomography, and contrast-enhanced pelvic magnetic resonance imaging favored adenomyosis with multiple leiomyomas; no definite gastrointestinal or pulmonary primary lesion was identified, and no obvious metastatic lesion was detected on chest imaging. Malignant entities including uterine sarcoma, endometrial carcinoma with myometrial invasion, and metastatic disease were considered preoperatively, but the predominantly intramyometrial distribution, lack of convincing diffusion restriction, absence of overt abnormal enhancement, and nonspecific clinical findings initially argued against malignancy. The patient underwent total abdominal hysterectomy with bilateral salpingo-oophorectomy and adhesiolysis. Histopathology demonstrated high-grade LCNEC arising predominantly within the myometrium, with extrauterine spread involving parametrial tissues, the right adnexa/mesosalpinx vasculature, and tumor deposits on the bladder surface. Immunohistochemistry showed strong neuroendocrine differentiation (synaptophysin+, chromogranin A+, CD56+) with a high proliferative index (Ki-67 approximately 80%), while PAX8 and hormone receptors were negative. Adjuvant chemoradiotherapy, further staging evaluation, and subsequent molecular testing were recommended but declined. Within months, the patient developed massive pelvic-abdominal wall recurrence with fistulization and radiologic evidence of multifocal invasion, and she died approximately 7 months after surgery. Conclusion: Intramyometrial uterine LCNEC can closely resemble adenomyosis and leiomyomas on imaging and may follow an explosive clinical course. This case highlights the diagnostic pitfalls of predominantly intramyometrial disease and underscores the importance of early suspicion, multidisciplinary reassessment when clinical and imaging findings are discordant, prompt platinum-based systemic therapy when feasible, and molecular profiling if the patient is willing to proceed.