Decreased bone growth arrest in weanling rats with multiple radiation fractions per day.

The relative growth arrest caused by fractionated irradiation delivered in single or multiple daily fractions was studied in weanling rats. Twenty-two day old male rats were treated to a total dose of either 20 or 25 Gy in five consecutive days to the distal femoral and proximal tibial epiphyses of the right and left leg. For each dose three treatment groups were followed for longitudinal tibial growth as measured on serial radiographs: (a) no treatment, (b) 5 fractions in 5 days or (c) 10 fractions in 5 days. Tibial length was significantly greater in the legs treated with twice-daily fractions (TDF) as compared with single daily fractions (SDF) with 23% and 27% sparing of growth arrest (at 200 days) in legs treated to total doses of 20 and 25 Gy respectively (p less than 0.001). This appeared to result from a continuously greater rate of growth during the first 40-50 days following TDF irradiation as compared with SDF. These data suggest that hyperfractionation may provide a means of reducing growth deficits in children when skeletal growth centers must be included in the irradiated volume.