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    Optimisation of neutron capture radiography for analysis of boron-10 in tissues
    (University of Technology, Sydney and Australian Nuclear Science and Technolog Organisation, 1991-01) Mrayati, H
    In Boron Neutron Capture Therapy (BNCT), a non—toxic compound of boron is selectively taken up by tumour tissue. The tumour is then irradiated by thermal neutrons, inducing the 10B(n, α)7Li fission reaction. These ions have a range of about one cell diameter, high linear energy transfer (LET) and result in the energy deposition of up to 2.3 MeV in the cancer cell. Consequently, reproductive cell death results, leading to the regression of the tumour reproduction. It is of critical importance to determine the spatial distribution of boron uptake in the tumour and this is achieved by Neutron Capture Radiography (NCR). The NCR system was characterised prior to its utilisation for boron—10 visualisation. NCR utilises a Solid State Nuclear Track Detector (SSNTD) to register the products of the fission reaction. These products cause structural damage to the SSNTD that can be revealed as pits after chemical etching with 6.25M NaOH at 70°C for 60 minutes. The pits can be observed by a light microscope. Thus, the boron—10 (10B) distribution in a biological section can be mapped by the location and density of pits. A polymer type of SSNTD known as Colombia Resin—39 (CR—39) is an alpha track detector and it was theoretically characterised in terms of: pit diameter (d), critical angle (ƟC), etching rate ratios (V), track (VT) and bulk (VB) etching rates, etching efficiencies (η), etching times (tE) and etch—induction time(tind) of alpha, lithium, and hydrogen ions. V and VB parameters were calculated from semi—empirical equations for etching condition T=70°C and etchant 6.25M NaOH. Their values are listed in the table below. Theoretical values were then compared with the experimental results. The calculated parameters can provide an estimation of the track parameters and thereby the optimum etching conditions for a NCR experiment. Error analysis was applied to both approaches. The measured and the published values of pit diameters of alpha are comparable with 5.2% variance. The measured bulk etching rate (VB is 10.5% higher than the calculated one. The calculated pit diameter of proton is comparable with the published value within 0.6%. The effects of high fluence neutron beam on the track background were investigated. It found that the background is proportional with neutron fluence. Fluence of order of 1012 n cm- 2 gives high track background and changes the optical characteristics of the detector. On the other hand, fluence of order of l010 n cm -2 or less gave a good track to background ratio. The results of CR—39 characterisation are utilised to determine the optimum etching. These conditions are essential for preferential visualisation of alpha from proton tracks in tissue samples. They are found to be: etchant concentration=6.25M NaOH, etching temperature=70C° and 60 minutes etching time, combined with thermal neutron fluence of order of 1012 n cm-2. Pits from different concentrations of 10B sample were photomicrographed and later were counted by using a light microscope to determine the lower and upper detection limits. The limits were found to be 2.5 and 80 parts per million (ppm) of boron - l0 solution respectively. The pit densities decrease as the etching time is increased. The spatial resolution of the technique is considered.to be 20 μm because of uncertainty in the origin of particle in tissue. The theoretical approach leads to the optimum conditions which were applied to reveal the spatial distributions of boron-10 in tissue samples. The conditions give a satisfactory result and demonstrate a differential 10B uptake by tumour tissue of nude mice. The maximum ratio of tumour pits to the adjacent muscle=32.l. Copyright ©1991 The Author.