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?

Most endometrial cancers have the endometrioid pattern and are classified as type I endometrial carcinomas. They arise in the setting of unopposed estrogen stimulation and may also have PTEN mutations as well as microsatellite instability. In contrast, type II endometrial carcinomas occur at an older age in the background of atrophic endometrium; they usually have a serous carcinoma pattern, but may also exhibit clear cell and mullerian mixed patterns, and TP53 mutations are common. Leiomyosarcomas and stromal sarcomas are far less common than endometrial carcinomas, and they have no known risk factors.