A 3-year-old girl from Delhi with unilateral retinoblastoma (right eye, International Classification Group C) has completed 3 cycles of neoadjuvant intravenous chemotherapy (vincristine, etoposide, carboplatin). MRI shows partial regression of the mass (50% reduction in volume), but the tumour remains intraocular with no extraocular extension. The child has light perception in the affected eye. What is the most appropriate next step in management?