The pig as a model for the study of radiation effects on the lung

The treatment of thoracic malignancies is frequently limited by the 'tolerance' of normal lung tissue. In order to learn more about the factors that influence lung tolerance an animal model that closely mimics the clinical exposure situation is required. The lungs of pigs are similar to those of man in a variety of ways and the animal's size permits the irradiation of partial tissue volumes comparable with those used clinically; very rarely in man is the whole lung irradiated. In this report, the available data for the effects of irradiation on pig lung are reviewed as they relate to the key issues in radiotherapy.

The dose-effect relationships for exposure to single doses indicate that for a significant impairment in both early and late lung function and for the histological detection of fibrosis, the dose-related changes in pig and man are similar. Studies with dose-fractionation using X-rays indicate a large dependence of the iso-effective dose on fraction number and fraction size, and the parameters obtained were not significantly influenced by the time of assessment after irradiation. A simple power-law function fitted the whole data set better than the linear-quadratic model, with a fraction number exponent (N) of 0.44+/-0.06 for treatments given in 1-30 fractions. The alpha/beta values ranged from 0.6 to 4.86 Gy, tending to increase with the length of the follow-up period; however; the majority of these alpha/beta values were not significantly different from zero at the 5% level. Studies of the effect of changes in the volume of lung tissue irradiated indicated the need for care in the use of the terms 'tolerance' and 'iso-effective' dose. Doses that were iso-effective for the severity of regional damage were not matched by those for total lung function. The same level of damage in a small volume compared with a large volume had less effect, i.e. was better tolerated in terms of changes in total lung function.

Iso-effective doses in pig and humans are lower than those for the more common laboratory animal species. This observation may be related to the differences in anatomical structure of the lungs in the different species.