Primary Alveolar Rhabdomyosarcoma of the Uterine Corpus Expressing MUC4 and OLIG2: A Case Report with Combined Morphological and Molecular Analysis

A 51-year-old woman with abdominal distention for 2 months presented with a mass lesion in the uterine corpus. Biopsy specimen obtained from uterus showed small round cell tumor, and cytologic examination revealed massive ascites with malignant cells. A total hysterectomy with bilateral salpingo-oophorectomy was performed. The tumor had replaced the entire wall of the uterine corpus, involving uterine cervix and bilateral adnexa. Microscopic examination revealed an alveolar pattern composed of primitive round cells with focal myoblast-like features. No carcinomatous component was identified. The immunohistochemical studies revealed positive staining of myogenic markers, myoD1, and myogenin. RT-PCR and Sanger sequencing of formalin-fixed, paraffin-embedded (FFPE) specimens identified PAX3-FOXO1 fusion gene. In the additional immunohistological studies of both biopsy and resected specimen, tumor cells showed MUC4 and OLIG2, which are recently reported surrogate markers to detect the fusion gene of rhabdomyosarcoma. We finally diagnosed alveolar rhabdomyosarcoma (ARMS) arising in the uterine corpus. ARMS is an extremely rare malignant tumor in the female genital tract, especially in the uterine corpus, and only 8 cases have been reported so far. This report documents the first case of molecularly-confirmed (fusion-positive) uterine alveolar rhabdomyosarcoma that expressed both MUC4 and OLIG2. In addition to delineating the molecular and prognostic characteristics, further studies are required to validate the diagnostic utility of MUC4 and OLIG2 in detecting the fusion-positivity status of ARMS arising in the uterine corpus.