Comments on "Analysis of Survival of C-18 Cells after Irradiation in Suspension with Chelated and Ionic Bismuth-212 Using Microdosimetry" by Stinchcomb and Roeske (Radiat. Res. 140, 48-54, 1994)

Abstract

We read with interest the recent paper by Stinchcomb and Roeske (1) on the microdosimetric analysis of C-18 cells irradiated by ac particles. The data used in their analysis were from Argonne National Laboratory experiments. These data have also been used in several intercomparison studies (2-4) of different oa-particle irradiation sources at the Pacific Northwest Laboratory (PNL), Case Western Reserve University and Argonne National Laboratory. We would like to make the following comments on the Stinchcomb and Roeske publication: 1. While we agree with the authors that the microdosimetry approach may provide more information than an average-dose approach (an accepted fact at our laboratory and others for a number of years), the purpose of the above-mentioned intercomparison was to determine the physical factors that affect dosimetry to compare biological data. These factors include: (a) different qualities and energies of radiation; (b) a-particle radioactivity from both parent and daughter radionuclides; (c) source-target geometry, which includes considerations of irradiation from radon and its progeny in the medium as well as intracellular localization of attached progeny; (d) cell and nucleus dimensions; (e) dose rates; and (f) length of exposure. These basic factors need to be resolved as the first step in any dosimetric approach. 2. An intercomparison study requires readily measurable parameters. The use of the cited parameter (Zo), defined as the specific energy deposited in the nucleus for which an individual cell has an average survival probability of l/e, is a more sophisticated concept, but it is obtained by derivation and is not directly measurable. The average dose concept (Do), which produces a 37% survival, is readily reproducible and easy to use as a point of comparison. To measure and verify the Zo concept, a method is required to deliver an exact dose (specific energy) to each nucleus of a large number of cells, and then follow them for survival analysis. We know of only a few institutions (including the microbeam facility at PNL) that have this capability. Initial experimentation with limited numbers of cells has begun with these facilities. 3. We have used the stochastic dosimetry approach at PNL to calculate probability distributions of specific energy and hit probability (5). More importantly, we have supported our hit-probability calculations with a biological measure of DNA strand breakage using single-cell gel lectrop oresis (6). In conclusion, we believe, as do the authors, that the microdosimetry approach brings additional understanding of radiobiological data, and we continue to use this approach in the analysis of data from our ongoing and collabora ive radiobiology studies.