Late radiation response of canine mediastinal tissues.

The mediastinal tissues which included heart, lung, trachea and esophagus of 70 adult beagle dogs were irradiated to a range of total radiation doses between 24 and 68 Gy given in 2, 3 and 4 Gy fractions. The purpose of the study was the calculation of alpha/beta ratios for morphologic and functional changes of the mediastinal tissues. Functional assays including echocardiography, electrocardiography, right heart hemodynamics and cardiac output were performed. Histomorphometric analyses of all tissues included in the field were done 2 years after treatment. Euthanasia was performed on 7 of 70 dogs prior to 2 years due to congestive heart failure and seven other dogs had signs of heart failure 2 years after treatment. Heart failure was thought to be caused by either pericardial effusions or constrictive pericarditis in these dogs. Heart failure occurred at doses of 62 and 68 Gy given in 2 Gy fractions, 60 Gy given in 3 Gy fractions and 52 Gy given in 4 Gy fractions. The ED50 values for pericardial fibrosis for 2, 3 and 4 Gy fractions were 46.1, 43.9 and 26.6 Gy, respectively. An alpha/beta ratio of 2.5 Gy was calculated by direct quantal response analysis. Small foci of myocytolytic lesions were detected in 11 dogs. Calculated ED50 values for myocytolysis were 70.4 Gy given in 2 Gy fractions and 50.8 Gy given in 4 Gy fractions. The estimated alpha/beta ratio was 3.2 Gy. Heart rates determined from physical examination and frequency of S-T segment changes increased with increasing dose. No other dose related changes were found in any of the other functional parameters. Functional changes were detected in the 14 dogs with clinical signs of heart failure. Focal consolidation and subpleural fibrosis were present in the irradiated lung volume. These late changes had no detectable physiologic effect in these dogs because of the small volume of lung irradiated. The ED50 values for lung consolidation were 54.3, 45.8 and 26.6 Gy after 2, 3 or 4 Gy fractions, respectively. The estimated alpha/beta ratio was 3.4 Gy. No dose-related changes could be detected in the trachea or esophagus at 2 years after treatment. These results demonstrate that lung and pericardium are the most responsive tissues in the mediastinum within the first 2 years after treatment. Myocardial lesions were present with high ED50 values, but were not found to be functionally significant at 2 years after irradiation. Human clinical data indicate that longer observation periods are needed for development of these lesions.(ABSTRACT TRUNCATED AT 400 WORDS)