Ultrasoft X rays have provided a valuable tool to probe subcellular mechanisms of radiation action. Sufficient data have become available to allow meaningful comparisons between laboratories and between biological assays. A brief summary is given of these data. They show that there is generally good agreement between the results from different laboratories but that there are apparently major differences in RBE between cell types which do not conform to simple expectations or explanations. This raises an intriguing conundrum which awaits satisfactory solution. The solution should provide valuable additional insight into the subcellular mechanisms, including information on spatial distributions of critical subcellular target material and/or essential difference between the responses of different cell types. A variety of potential solutions to the conundrum are discussed and it is shown how Monte Carlo track structure analyses allow some of these to be studied in terms of properties of radiation tracks over nanometre dimensions.
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Goodhead et al. (1990) studied this question.