Effects of boron neutron capture irradiation on the normal lung of rats

The whole lung of rats was irradiated with X-rays, thermal neutrons, or thermal neutrons in the presence of p-boronophenylalanine (BPA). A 20% increase in breathing rate, in the period 40–80 days after irradiation, was indicative of radiation-induced pneumonitis. The ED50 (±SE) for a 20% increase in breathing rate, relative to age-matched controls, was 11.6±0.13 Gy for X-rays and 9.6±0.08 Gy for neutrons only. This indicated a thermal neutron beam RBE of 1.2 and an RBE of 2.2 for the high-LET components of the dose, assuming a dose reduction factor of 1.0 for gamma rays. Preliminary data indicate the compound biological effectiveness factor for BPA in the lung is approximately 1.5.