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Heterogeneous Distribution of Technetium-99m-Labeled Microspheres in Rat Lungs: Microautoradiographic Evidence and Dosimetrie Consequences

Department of Biophysics, School of Medicine, Xavier Bichat University, Paris; Radioisotopes Department, Montfermeil Hospital, Montfermeil, France

Correspondence: For correspondence or reprints contact: N. Colas-Linhart, PhD, Faculté de Médecine Xavier Bichat, Laboratoire de Biophysique, BP 416, 75870 Paris, Cedex 18, France.

ABSTRACT

The heterogeneity of ^99mTc-labeled microspheres distribution within rat lung was visualized and quantified using a microautoradiographic "track" method (MAR). Methods: MAR was used to study the uptake of radioactivity by individual microspheres, thereby enabling calculation of the range of particle activity. MAR was also used to visualize in rat lung sections the intrapulmonary distribution of the microspheres within the lungs after intravenous administration. The mean doses delivered to the cells in close contact with the labeled microspheres were calculated taking only the ^99mTc electron emissions into account. Results: All the microspheres were labeled. Nevertheless, the spectrum of visible tracks varied by a factor of 10, inducing a variable activity per microsphere from <36 Bq to 325 Bq (mean activity = 94 Bq/microsphere). No correlation existed between the radioactivity uptake and the size of microspheres. A very heterogeneous tridimensional distribution of the microspheres within the lungs were demonstrated with interparticle distances ranging from 57–4400 µm. On the other hand, only 1 of 2000 rat lung capillaries was obstructed. Using the mean activity, calculated delivered doses were found to reach approximatly 6 Gy for the closest endothelial cells and 2 Gy for epithelial cells. However, such high doses were delivered to only a few cells. Conclusion: The number of obstructed capillaries in human lungs is lower than in rat lungs; the distances between microspheres should be larger. Nevertheless, the individual doses absorbed by the pulmonary cells closest to the microspheres should be very important.

Key Words: microautoradiography • technetium-99m-labeled microsphere • cellular dosimetry • rat lungs

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