MIBG is selectively concentrated in neuroblastoma cells, and radioiodinated MIBG has been used with some success for targeted radiotherapy. However, long-term cure remains elusive, and the topoisomerase I inhibitor topotecan may improve upon existing [131I]MIBG therapy. While synergistic killing by combinations of ionising radiation and topoisomerase I inhibitors has been reported, there is no consensus on optimal scheduling. Furthermore, there has been no attempt to demonstrate radio-potentiation by topoisomerase I inhibitors and targeted radiotherapy. We are investigating various scheduled combinations of topotecan and [131I]MIBG on neuroblastoma cells, and preliminary data suggests that topotecan induces increased accumulation of [131I]MIBG in vitro.