The LD/50 between 40 and 180 days after irradiation of the thorax has been taken as a measure of lung damage in mice. The method has been used to derive the relationship between total dose, number of fractions (N) and overall treatment time (T). For X rays the results fit an Ellis type of relationship, i.e. total dose alphaN0-25 T0-07. This only applies for more than 8 fractions, below which N0-39 becomes a good fit to the results. For neutrons, the best fit is total dose alphaT0-07. Various possible explanations for the T factor are discussed, but for lung damage we attribute it to a slow repair process which possibly may only be observed in slowly proliferating tissues. Slow repair is about 100 times less rapid than Elkind-type of repair of sublethal damage and whereas Elkind repair is greatly reduced for irradiation with fast neutrons, relative to X rays, slow repair is not.

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The relative biological effectiveness (RBE) for damage to mouse lung was measured for single doses and up to 30 fractions of 8 MV x rays and fast neutrons, relative to 250 kVp x rays. With 8 MV x rays the RBE was 0.87 and did not vary with dose per fraction between 150 and 1200 rad. With fast neutrons the RBE did not vary from 1.5 at 1200 rad of x rays, which is about 30% lower than that for skin, to 3.7 at 150 rad of x rays, which is about 10% less than that for skin. The implications for treatment of the lung with fast neutrons are discussed.