Study of a lung-equivalent material for BNCT dosimetry

In the last years, noticeable interest has grown up regarding lung cancer treatment. Nowadays, lung has been considered a candidate for boron neutron capture therapy (BNCT) application. BNCT is a radiotherapy still in study that takes advantage of the possibility of selectively accumulating the isotope 10B in tumour cells and of the high cross section (3837 b) of the reaction with thermal neutrons 10B(n,α)7Li. The short range in tissue of the emitted α and 7Li particles (<10 m) allows localised energy release in tumour cells, saving the surrounding healthy ones. The high linear energy transfer (LET) and relative biological effectiveness (RBE) of the emitted α and 7Li particles makes BNCT to be potentially effective for radio-resistant tumours. Moreover, owing to the selective concentration of 10B in tumour cells, BNCT is particularly promising for diffused tumours. In order to perform reliable dosimetry aimed at validating treatment planning calculations, it is necessary to design suitable phantoms having good tissue-equivalence for neutrons and for every secondary radiation. A proper study has been carried out to develop a matrix having proper characteristics to simulate lung tissue. According to ICRU 44, the average lung density of an adult men is 0.35 g/cm3. A tissue-equivalent material with low-density has been obtained by preparing a gel matrix with Agar or gelatine, added with a surfactant (sodium-dodecyl-sulphate SDS-L4390). The proper procedure has been set up, in order to attain the density of 0.35 g/cm3. The produced material has been studied in order to investigate the correctness of its properties. Material homogeneity and both mass and electron densities have been measured by means of Computer Tomography. Neutron transport has been evaluated by means of Monte Carlo simulations, using the code MCNP-X. The results have confirmed the lung equivalence of the proposed matrix. A thorax phantom has been designed, composed of layers of Poly(methyl methacrylate) (PMMA) and polyethylene suitably shaped in order to compose a thorax portion containing a simulated lung. This thorax portion can be faced at the collimator of the BNCT epithermal column of a nuclear reactor, to perform the necessary dosimetry experiments. In-phantom dose and thermal neutron fluence measurements with TLDs and images with gel dosimeters are in program.