HISTOLOGICAL CHANGES AFTER IRRADIATION 1129 



Urinary Bladder. There are but few reports on the reaction of this 

 organ to irradiation. The epithelium is resistant, although with high 

 doses used in man for therapy of lesions in neighboring organs, desquama- 

 tion of epithelium has been observed. The most marked changes involve 

 the connective tissue in which there is an early transient erythema and a 

 second erythema occurring some three or four weeks later. Ulceration 

 of the mucosa has been reported. Experimental studies are needed. 



RESPIRATORY SYSTEM 



The minute structure of the respiratory portion of the normal mam- 

 malian (and avian) lung is one of the unsolved problems of normal 

 histology. The problem revolves around the delineation of the struc- 

 tures intervening between the vast network of blood vessels carrying 

 blood for aeration and the air spaces. Linked with these histological 

 uncertainties are the difficulties of analyzing mild changes resulting from 

 any irritation and the frequent occurrence of inflammation in the lungs 

 of laboratory animals and man. The analysis is further complicated by 

 the relative radioresistance of the organ, it being decidedly less sensitive 

 than any hematopoietic organ, and by the intervention of a latent period 

 of two or three weeks. Part of the divergence of opinion in the literature 

 has resulted from differences in the intervals between divided doses. 



Just as an erythema dose delivered to the skin has a slight transient 

 effect, so an equivalent dose delivered to the lung has a definite, though 

 again a transient, effect. Severe damage to the lung, however, usually 

 occurs only after the repeated doses of radiation customary for therapy, 

 radiation pneumonitis being a frequent complication of such treatment. 



In an extensive study on rabbits receiving 1500 to 9000 r Engelstad 

 (1934) described four main stages in the pulmonary reaction: 



1. An initial period with acute degenerative changes in the lymphatic 

 tissue of the lung, increased mucus production in the conducting passages, 

 hyperemia, and some edema with mild leukocytic infiltration in the 

 stroma and respiratory portion. The first changes begin 2 hours after 

 irradiation. 



2. A latent period of two to three weeks. 



3. Principal reaction with degenerative changes in the bronchial epi- 

 thelium and acute bronchopneumonic changes about the bronchi and 

 blood vessels. There is also a great increase in alveolar macrophages, 

 often forming giant cells. This stage may last one to two months and 

 gradually regresses. 



4. Predominantly regenerative stage with proliferation of connective 

 tissue, with sclerosis (at times with bone formation) and slight prolifera- 

 tion of bronchial epithelium. 



