The effect of low dose-rate and cyclophosphamide on the radiation tolerance of the mouse lung.

Mice were given cyclophosphamide 30 mg/kg intraperitoneally before thoracic irradiation at high dose-rate (HDR, 100 cGy/min) or low dose-rate (LDR, 5 cGy/min). The development of pneumonitis was monitored by regular measurement of breathing rate. Peak rises in breathing rate occurred around 4-9 weeks in those given cyclophosphamide before irradiation, and around 14-16 weeks in those given radiation alone, regardless of dose-rate. The dose reduction factor (DRF = LDR/HDR ratio) for LDR irradiation was congruent to 1.8 but LDR sparing was abolished (DRF congruent to 1.0) when cyclophosphamide was given before irradiation.