Abdominopelvic Metastasis of Endometrial Serous Carcinoma Initially Misdiagnosed as Early-Stage Low-Grade Endometrioid Carcinoma: The Importance of Recognizing Minimal Uterine Serous Carcinoma

Minimal uterine serous carcinomas (MUSCs) include serous carcinomas with invasion confined to the endometrium (superficial serous carcinoma) and those without stromal invasion (serous endometrial intraepithelial carcinoma). Although these tumors are confined to the endometrium proper, they have highly metastatic potential for disseminating to extra-uterine sites. We report here a case of MUSC that was initially misdiagnosed as early-stage low-grade endometrioid carcinoma but later metastasized to the abdominopelvic peritoneum. The patient was a 61-year-old woman who was diagnosed with grade 1 endometrioid carcinoma of the endometrium and underwent total hysterectomy. Because the tumor was confined to the endometrium (International Federation of Gynecology and Obstetrics stage IA), no further treatment was performed. However, several metastatic tumor masses were detected in the vaginal stump and abdominopelvic peritoneum 7 years after the surgery. Histologically, the metastatic tumor tissues showed high-grade carcinoma. A review of previous hysterectomy slides showed multiple separate foci of atypical glandular proliferation measuring up to 0.8 cm in the greatest dimension and consisting of markedly atypical cells involving the surface and atrophic glands. The tumor showed a predominantly glandular architecture without evident papillary growth or stromal invasion. However, it had large, pleomorphic nuclei showing a high nuclear-to-cytoplasmic ratio, conspicuous eosinophilic nucleoli, and numerous mitotic figures. Characteristically, the tumor showed marked nuclear atypia immediately appreciated at low magnification in the background of well-formed glandular structures, indicating a significant discordance between nuclear and architectural features. On immunostaining, both the uterine and metastatic tumor tissues exhibited diffuse and strong p16 expression and mutant pattern of p53 expression, confirming the diagnosis of serous carcinoma. In summary, the case findings support that failure to preoperatively recognize high-risk endometrial carcinoma is associated with worse outcomes. Complete surgical staging and accurate pathological diagnosis are critical for patients with serous carcinoma even at the early clinical stage.