A 62-year-old woman from Mumbai with a history of unopposed estrogen exposure (never used hormone replacement therapy, nulliparous, BMI 31 kg/m²) presents with postmenopausal bleeding. Endometrial biopsy shows grade 3 endometrioid adenocarcinoma. Staging MRI reveals invasion beyond the outer third of the myometrium (stage IIIA) with no cervical stromal involvement or distant metastases. After TAH-BSO and pelvic lymph node dissection, final pathology confirms stage IIIA disease. Pelvic lymph nodes are negative. What is the most appropriate adjuvant treatment?