Uterine Carcinosarcoma With an Adenosquamous Carcinoma and Low-Grade Endometrial Stromal Sarcoma-like Component: A Case Report

Background/Aim: Uterine carcinosarcoma, also known as malignant mixed Müllerian tumor, is currently regarded as a metaplastic carcinoma. The sarcomatous component usually consists of homologous or heterologous high-grade mesenchymal elements, whereas low-grade endometrial stromal sarcoma (LG-ESS) has not been recognized as a conventional component of uterine carcinosarcoma. This is a case report of an exceptionally rare biphasic uterine malignancy composed of an adenosquamous carcinoma and an LG-ESS-like stromal component with an extensive morphological transition. Case Report: A 55-year-old nulliparous woman presented with an endometrial mass involving the myometrium and the endocervix. Endometrial biopsy revealed atypical hyperplasia, and cervical punch biopsy showed adenosquamous carcinoma. Subtotal hysterectomy revealed a high-grade endometrioid carcinoma with extensive malignant squamous differentiation (descriptively termed adenosquamous carcinoma in this report). The epithelial component gradually transitioned into diffuse stromal proliferation, which was morphologically consistent with LG-ESS. The stromal component exhibited infiltrative myometrial growth, extensive lymphovascular invasion, diffuse CD10 and vimentin expression, and loss of epithelial markers, whereas the epithelial component showed CK7, PAX8, CEA, and focal estrogen receptor expression. Squamous cell carcinoma, LG-ESS-like stromal component, and transitional area showed block-type p16 positivity and wild-type p53 staining pattern. Postoperatively, the patient received concurrent chemoradiotherapy but experienced rapid disease progression with pelvic recurrence and metastatic lymph node enlargement. Targeted next-generation sequencing revealed microsatellite stability and wild-type BRAF. Conclusion: Unlike previously reported synchronous or collision tumors composed of endometrial carcinoma and LG-ESS, this tumor demonstrated a continuous morphological and immunophenotypic spectrum linking the epithelial and stromal components. Although molecular confirmation of clonality was unavailable, the findings favored a single biphasic neoplasm with divergent differentiation rather than two independent tumors. This case expands the currently recognized morphological spectrum of uterine carcinosarcoma and highlights the potential role of epithelial–mesenchymal transition in generating an LG-ESS-like stromal component.