125Iodine decay in DNA: a discussion of its effectiveness for the breaking of DNA strands.

125I incorporated in DNA is known to be exceptionally toxic. Values of D0 range from about 40 to about 90 decays for survival of mammalian cells. The effectiveness of 125I in DNA with respect to the induction of breaks of the DNA strands, however, appears to be comparatively low. The numbers of strand breaks per energy deposited in subnuclear cellular structures such as DNA is smaller for a disintegration of 125I than for gamma-rays. The difference in effectiveness diminishes with increasing mass of the considered sensitive volume. The apparent inefficiency of 125I-decay may, on one hand, result from a waste of local energy deposition. On the other hand, it may be caused by a multitude of local strand breaks (clusters) induced by 125I-decay which are measured as one break only by the conventionally applied techniques of strand break measurement. The apparent inefficiency of 125I may be evidence furthermore for the importance of considering not only the DNA as the sensitive target but with equal pertinence the gross sensitive volume, i.e. the whole cell nucleus. Further, for drawing meaningful comparisons, it may be necessary to take into consideration the microdosimetric event size distributions for the critical targets.