The microscopic biodistribution of radioactivity in tissues is important in determining microdosimetry. This study addresses the use of frozen section microautoradiography in studying the subcellular distribution of 111In in leukocytes labeled with 111In-oxine.
Methods: In conjunction with frozen section microautoradiography, computer image analysis methods were applied to the analysis and quantification of leukocyte sections and superimposed autoradiographs. Rapid cell fractionation was used to confirm the results.
Results: The emulsion (Ilford K2) response was linear over the concentration range investigated (0-33 MBq ml-1). Resolution of radionuclide distribution was better than 2 microns. The autoradiographs showed no dependence of radiolabel uptake on cell type. Classification of all cells into intervals according to grain density suggests an exponential rather than normal distribution, with approximately 50% of cells having little or no radiolabel. In any one sample, cells which were heavily labeled were approximately 10 times more likely to be found in aggregates (60% found in aggregates, mostly neutrophils) than cells which were not heavily labeled (6% found in aggregates); and the grain densities were at least twofold higher over nuclei than over cytoplasm. The last observation was confirmed by the rapid cell fractionation method which showed that approximately 57% of the total radioactivity was bound to nuclei.
Conclusion: Frozen section microautoradiography is a practical and reliable approach to determining sub-cellular distribution of 111In. The radiolabeling process causes aggregation of neutrophils. Uptake is not significantly dependent on cell type, but only a fraction of cells are appreciably labeled. The radioactive concentration in cell nuclei is at least two-fold higher than in cytoplasm. Microautoradiography can be used to provide distribution data as input into computer models for sub-cellular dosimetry.