77 Advanced-Stage Uterine Mesonephric-Like Adenocarcinoma Without Endometrial Involvement: A Diagnostic Challenge

Abstract Introduction/Objective Mesonephric-like adenocarcinoma (MLA) is a rare and recently described subtype of gynecologic carcinoma, most commonly arising in the endometrium or ovary. These tumors exhibit mesonephric morphology and immunophenotype but without a clear embryologic origin from mesonephric remnants. They are typically hormone receptor-negative and present at advanced stages. Diagnosis may be especially challenging in post-treatment specimens, where chemotherapy-induced changes can obscure the primary site. Methods/Case Report We report the case of a 75-year-old woman who initially presented with lower abdominal and groin pain. Imaging revealed fixed inguinal lymphadenopathy and a heterogeneous mass centered in the uterine fundus. A biopsy of the left inguinal lymph node demonstrated metastatic carcinoma, consistent with a Müllerian primary. Subsequent endometrial biopsy revealed scant strips of inactive endometrium. The patient was treated with platinum-based chemotherapy, which resulted in stable disease on restaging imaging and clinical improvement. She later underwent a total abdominal hysterectomy with bilateral salpingo-oophorectomy, excision of a pelvic wall nodule, and omentectomy. Histologic examination revealed mesonephric-like adenocarcinoma (MLA) extensively involving the myometrium and associated with adenomyosis. The tumor showed focal extension to the uterine serosa and involved the pelvic sidewall and omentum. Microscopic foci of tumor were also identified in the left ovary. Notably, no tumor was identified within the endometrium, even after entire submission of the endometrium. Immunohistochemical studies showed tumor cell positivity for PAX8, GATA3, CD10 (luminal), and focal TTF-1. The tumor was negative for estrogen receptor, progesterone receptor, and Napsin A. An inverse staining pattern of GATA3 and TTF-1, characteristic of MLA, was observed. P53 exhibited wild-type staining, and p16 showed a patchy staining pattern. Results NA Conclusion Although no tumor was identified in the endometrium, the absence of tumor in the cervix, predominant myometrial infiltration, and limited serosal extension (without an outside-in growth pattern) favored a uterine origin. The absence of tumor in the endometrial cavity may be attributed to the effects of neoadjuvant chemotherapy. However, given the association with adenomyosis, an origin from adenomyotic foci cannot be entirely excluded. Additionally, microscopic foci of tumor identified in the left ovary are consistent with metastatic disease. MLA is a rare and aggressive endometrial carcinoma subtype that often presents at an advanced stage with poor prognosis. Recognition of this entity is essential, especially in atypical presentations such as this case, to ensure accurate diagnosis and appropriate management.