A comparison of neutron rbe values for normal canine lung by densitometry,pulmonary function and radionuclide studies

Thirty-nine adult male beagles received either fast neutron or photon irradiation to the right thorax. there were two unirradiated control dogs. Twenty-four dogs received fast neutrons with a mean energy of 15 MeV to total doses of 1000, 1500, 2250, or 3375 rad delivered in four fractions per week for six weeks. Fifteen dogs received total doses of 3000, 4500, or 6750 rad of photons (⁶⁰Co) in the same fractionation pattern. Radionuclide evaluations of pulmonary function were performed pre-irradiation and every three months post-irradiation for two years. These included: (1) radioaerosol deposition of ^99mTc-phytate, an insoluble radiocolloid, (2) ¹³³Xe ventilation studies and (3) ^99mTc-macroaggregated albumin perfusion images. A chest radiograph taken prior to each radionuclide evaluation was measured for density changes. Mechanical properties of pulmonary function were studied pre-irradiation and at 3-month intervals post-irradiation for one year. Values for the relative biological effectiveness (RBE) of fast, neutrons in producing changes in these parameters were obtained by plotting the changes from pre-irradiation values in the right lung as a function of the total dose. The RBE for neutron damage to normal lung tissue was always greater than 4 in the clinical dose range of 4000–6000 rad of photons, regardless of the parameter that was used to generate the value.