The Range of High LET Effects from 125 I Decays

Abstract

Track structure technique are applied to calculate energy depositions in cylindrical targets 20 .ANG. in diameter (simulating the DNA duplex) containing, or near, 125I decays. Two problems are examined: (1) The possible effects of incorporated versus nonincorporated 125I are evaluated; (2) the extent of the radiological damage along the DNA is described and discussed for individual decays taking place in the DNA. The results of three different calculations are presented: (1) The distribution of the total energy deposited in the target per decay: Here it is shown that the 125I decays deposit considerably more energy than 5-MeV .alpha. particles when the decay occurs on the central axis of the cylinder. When the decay occurs at 40 .ANG. from the axis, the energy depositions are small and infrequent, showing that the iodine decay must occur within this distance to produce a high LET-like effect. (2) The distribution of average energy depositions around a curved cylinder simulating the DNA duplex encircling the nucleosome. There is a rapid decrease in the energy deposited in elements (of size resembling a base pair) away from the location of the decay. At .apprx. 17 .ANG. (.apprx. 5 bp) from the decay the mean energy deposited in an element is reduced by a factor of 10. (3) The energy deposited in individual elements of the cylinder is presented for single decays: The smooth decrease in average energy depositions with distance from the decay ((2) above) is not reflected in individual decays.