A 58-year-old postmenopausal woman presents with abnormal uterine bleeding. Endometrial biopsy reveals grade 1 endometrioid adenocarcinoma confined to the endometrium (stage IA). She is medically fit and undergoes total abdominal hysterectomy with bilateral salpingo-oophorectomy. Histopathology confirms no myometrial invasion, no lymphovascular space invasion, and negative pelvic lymph nodes. What is the drug of choice for adjuvant chemotherapy in this patient?