A 42-year-old woman with metastatic breast cancer presents to the palliative care unit with severe, uncontrolled cancer pain (numerical rating scale 9/10) despite being on oral morphine 60 mg twice daily. She has no nausea, normal renal function (eGFR 92 mL/min/1.73m²), and no signs of opioid toxicity. Her pain is localized to the right chest wall where the primary tumor is located. What is the most appropriate next step in pain management?