Repair in mouse lung between multiple small doses of X rays.

Multiple fraction experiments have been carried out to determine the response of mouse lung to repeated small doses of 240 kV X rays down to 150 rad/fraction using breathing rate and lethality to assess damage. Two experimental approaches were used to measure the effect of small doses in vivo: (1) multiple equal doses and (2) multiple priming doses followed by a large test dose. Analysis was performed using the multitarget two-component model and the linear quadratic model of cell survival. The amount of repair was calculated as a function of either dose per fraction (FR) or total dose (Frec). Both FR and Frec increased with decreasing dose per fraction but the change in FR was small. The advantage of Frec was that it varied more rapidly with dose per fraction than FR, so that possible differences between tissue repair capabilities are more visible on plots of repair as a function of dose per fraction. FR and Frec both decreased with the level of single-dose isoeffect injury; thus neither parameter is acceptable for comparing repair capability of different normal tissues with widely differing single-dose end point levels. Beta/alpha values were calculated and found to be a more acceptable index of repair capability than either FR or Frec because unlike those two parameters, beta/alpha varied little with level of damage. Beta/alpha values of 1.7 to 4.2 krad-1 were obtained for both lung death and increased breathing rate and are clearly intermediate between the lower beta/alpha ratios for acute reactions, i.e., skin and intestine, and the higher values for late reactions in kidney and spinal cord.