Methodological developments for application to the study of physiological boron and to boron neutron capture therapy

The combination of immunogold labelling with electron microscopy or the dir ect detection of boron by electron energy loss spectrometry have the best l ateral resolution for the imaging of boron or boron binding sites in tissue s at the sub-cellular level. However these methods do not discriminate the boron isotopes. A number of physical methods make it possible to combine an alytical imaging with isotopic labelling for boron studies in biological ma terial. Secondary ion mass spectrometry has the potential to isotopically l ocalise virtually any element with a resolution of similar to 250 nm with c onventional instruments and 20-50nm with prototype instruments or with the NanoSIMS50; although SIMS has a relatively poor sensitivity for boron detec tion in biological matrices, boron imaging in plant samples is possible. La ser microprobe mass analysis also has the potential to detect boron isotope s with a lateral resolution of 3 to 5 Pm and a detection limit of a few ten s of mug/g with the conventional instruments and of the order of 1 ng/g wit h the new LARIMP system; although mass resolution of LMMS is in general not very good, the risk of interference by other ions at the level of boron ma sses is limited. Neutron capture radiography is probably the easiest techni que for boron imaging and boron isotopic labelling studies in tissues and s ometimes at the subcellular level, although it detects only B-10 isotopes. Nuclear reactions with charged particles (nuclear reaction analysis) have t he potential to detect both isotopes of boron and carry out absolute boron concentration measurements with minimal matrix effects, limited risk of int erference by other nuclides, a lateral resolution of a few mum at the best, a detection limit better than 1 mug/g for B-11, of the order of 10 mug/g f or B-10 and an accuracy of 1 to 2% in the determination of B-10/B-11 isotop ic ratios. Preventing the diffusion of possibly mobile forms of boron durin g the preparation of the biological specimens is still a difficult problem for most techniques. The appropriate application of those methods, or their mutual combination or combination with other methods has made it possible: i) to yield information about the boron concentrations and fluxes in sub-c ellular compartments and support the view that the cellular transport of bo ron was mainly passive under the experimental conditions under consideratio n; ii) to image the distribution of boron and of boron binding sites in tis sues and sometimes at the sub-cellular level; iii) to study the short-dista nce diffusion and the long-distance transport of boron in plants and to ass ess the role of the phloem in the long-distance transport in various plant species; iv) to determine the origin (seed reserves vs uptake by roots) of the boron present in different sub-cellular compartments. For boron neutron capture therapy of cancers, invasive techniques of boron detection and ima ging are comparable to the techniques described above for the study of phys iological boron; for clinical applications, non-invasive techniques to foll ow B-10-compounds in vivo are being developed, especially by targeting of s uch compound by F-18 and the use of positron emission tomography or by dire ct detection of B-10 by magnetic resonance imaging.