A 58-year-old woman came to the gynecological clinic. She complains of bloody discharge from her genital tracts. Menopause is 8 years. Gynecological examination: the uterus is slightly enlarged, dense to touch with limited mobility: the uterine appendages cannot be detected; parametrium is free. Fractional curettage of the uterine cavity yields a significant amount of medullary substance in the scrape. What is the most likely diagnosis?