A well-characterized in vitro cryogenic preparation for ion microscopic isotope imaging, which minimizes redistribution of diffusible species, was used to determine the distribution of boron in GS-9L gliosarcoma cells incubated with the boron neutron capture therapy agent, p-boronophenylalanine (BPA). At the subcellular level, boron from BPA distributes relatively homogeneously within the glioma cell. Boron from BPA was eliminated rapidly, indicating that most is unbound. Thus a large pool of boron is susceptible to diffusion artifact. Removal of this artifact increases the degree of confidence in microdosimetric results inferred from the homogeneous subcellular distribution. The ion microscopic imaging of boron in subcutaneous tumors cryofixed in situ was achieved in rats treated with BPA. Boron signals from BPA were adequate to image microdistributions at the 1-μm resolution level. As in the in vitro case, boron did not localize discretely at the subcellular level. However, boron heterogeneity was seen at the tissue level. Physiologically valid cellular potassium and sodium levels were seen, which demonstrates minimized redistribution artifact. Future tissue studies designed to correlate ion microscopic boron images to microscopic structure are feasible using cryogenic sample preparation and ion microscopy.

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