A 52-year-old postmenopausal woman presents with postmenopausal bleeding and pelvic pain. Transvaginal ultrasound shows an endometrial mass with myometrial invasion up to the inner third. MRI confirms FIGO stage IB endometrial cancer. She undergoes total abdominal hysterectomy with bilateral salpingo-oophorectomy and pelvic lymph node dissection. Histopathology reveals grade 2 endometrioid adenocarcinoma with lymphovascular space invasion (LVSI). Pelvic lymph nodes are negative. Which of the following is the most appropriate adjuvant treatment?