A study of patients with postmenopausal uterine bleeding reveals that some of them have malignant neoplasms that arise from prior atypical hyperplastic lesions. The peak incidence is between 55 and 65 years of age in women who have obesity, hypertension, and/or diabetes mellitus. Molecular analysis reveals mutations of the PTEN tumor suppressor gene in most of them. Their malignancies tend to remain localized for years before spreading to local lymphatics. Which of the following neoplasms is most likely to have these characteristics?