A 3-year-old girl from Delhi is found to have bilateral retinoblastoma on screening — the right eye has a Group C tumour (medium-sized, no vitreous seeding) and the left eye has a Group B tumour (small, no vitreous involvement). Systemic staging (chest CT, abdominal ultrasound, bone marrow biopsy) shows no metastases. Intraocular pressures are normal bilaterally. The parents ask about treatment options. What is the most appropriate next step in management?