A 32-year-old woman from Delhi underwent suction evacuation 6 weeks ago for a histologically confirmed complete hydatidiform mole. She had an uncomplicated post-evacuation course. Today she presents with persistent vaginal bleeding and mild lower abdominal pain. Her β-hCG at evacuation was 280,000 mIU/mL; repeat β-hCG today is 18,000 mIU/mL. Pelvic ultrasound shows a heterogeneous mass within the myometrium (2 cm × 1.5 cm) with colour Doppler showing increased vascularity. What is the most appropriate next step in management?