Radiobiological considerations in the treatment of neuroblastoma by total body irradiation.

Neuroblastoma is a radiosensitive neoplasm for which total body irradiation (TBI) is presently under clinical consideration. Collated data on the radiobiology of human neuroblastoma cells in vitro (11 cell lines derived from seven patients) indicates moderate cellular radiosensitivity and low capacity for accumulation of sublethal damage (median survival curve parameters: Do = 104 cGy, Dq = 32 cGy, n = 1.36). Mathematical studies incorporating these parameters suggest that low dose fractionated TBI is unlikely to achieve significant levels of tumour cell kill. When high dose TBI is used in conjunction with bone marrow rescue a tumour "log cell kill" of 4-5 should be achievable. This effect would be additional to that achieved by chemotherapy. The optimum schedule for exploitation of radiobiological differences between neuroblastoma cells and the dose-limiting normal tissues has a hyperfractionated structure. Twice-daily treatments with fraction sizes in the region 120-150 cGy seems appropriate. Single dose treatments at high dose rate are contraindicated. Fractionated TBI with bone marrow rescue may be curative for some patients in clinical remission who are presently destined to relapse.