The radiobiological basis for a possible improvement of the differential effect with multiple daily fractionation

What is of interest in this paper is whether the different biological effects of multiple daily fractionation will improve upon the efficacity of classical fractionation. This problem may be approached from two aspects. Does treatment with multiple daily fractions of less than 2 Gy necessitate an adjustment of the total treatment dose. It this is so, does M.D.F. improve the differential effect; does it improve tumor control without increasing normal tissue damage. To answer these questions, the iso-effect curves for several experimental systems in which the total dose required to produce a given biological effect was plotted against the number of fractions were studied. The data are still insufficient to draw definitive conclusions. But, it seems that for acute effects and for certain tissues for which the isoeffect curves reach a plateau at about 2-3 Gy, treatment with multiple daily fractions of less than 2 Gy does not necessitate an adjustment of the total treatment dose and does not influence the differential effect for tissues showing the same plateauing curve. However, for late normal tissue tolerance and other systems for which certain observations suggest an isoeffect curve which continues to increase with decreasing dose per fraction, the use of fractions of less than 2 Gy may decrease late radiation effects for the same level of early tolerance and may increase the differential effect when compared to systems with a ''plateauing'' curve. The variation in the isoeffect dose is probably of the order of 10%