MMR deficient dedifferentiated endometrial carcinoma with synchronous uterine leiomyosarcoma: a rare collision tumour-case report and literature review.

INTRODUCTION: Dedifferentiated endometrial carcinoma (DEC) is a rare and aggressive uterine malignancy composed of low-grade endometrioid carcinoma admixed with an undifferentiated component and is associated with a poorer prognosis than conventional high-grade endometrioid carcinoma. Its synchronous occurrence with uterine leiomyosarcoma as a collision tumour is exceptionally rare. CASE PRESENTATION: A 74-year-old postmenopausal woman presented with abnormal uterine bleeding. Imaging revealed an endometrial mass, and she underwent total abdominal hysterectomy with bilateral salpingo-oophorectomy. Histopathological examination demonstrated DEC with subclonal mismatch repair (MMR) deficiency and a morphologically distinct leiomyosarcoma. Immunohistochemistry showed loss of MLH1 and PMS2 expression in approximately 40% of tumour cells within the dedifferentiated component, with retained expression in the low-grade component. MLH1 promoter hypermethylation analysis demonstrated a methylation level of 55%, supporting a sporadic origin. A morphologically and immunophenotypically distinct uterine leiomyosarcoma was identified within the same specimen, consistent with a true collision tumour. The diagnosis was confirmed through comprehensive morphological, immunohistochemical, and molecular analyses. Postoperative imaging demonstrated nodal disease, and the patient subsequently received adjuvant therapy. At 1-year follow-up, she remained in complete remission. CONCLUSION: This rare collision tumour highlights the importance of integrated histopathological and molecular evaluation, particularly in recognising subclonal MMR deficiency and tumour heterogeneity.