Dose enhancement in fast neutron tumour therapy due to neutron captures in 10B.

High energy neutrons, applied in fast neutron tumour therapy, lose energy when passing through tissue and are at the end of their trajectories captured in nitrogen, hydrogen or other normally occurring elements. If the tissue contains 10B, which has a very high cross section for capture of thermal neutrons, then disintegration products of this process, helium and lithium ions, give a dose enhancement which, if the boron is targeted to tumour cells, may be beneficial. The dose enhancement was in the present study calculated as a function of the 10B concentration in the cells and as a function of different thermal neutron fluencies. If the tumour cells contained 10 or 100 microns 10B/g the average dose enhancement was about 20 or 200 mGy respectively. This was obtained with the thermal neutron fluency 2.0 x 10(10) n/cm2. The relative biological effectiveness of the neutron capture process is unknown but assuming the factor 2, these doses correspond to 0.04 or 0.4 CGE (cobolt-60 gray equivalent) respectively, which could directly be compared to the 2-3 Gy of low-LET radiation that is daily applied in conventional radiotherapy. However, if thermal or epithermal neutron fields are directly applied to the patients a hundred times higher thermal neutron fluency can be used. This gives, in the cases with 10 or 100 micrograms 10B/g, about a hundred times higher average doses so that 2-20 Gy, corresponding to about 4-40 CGE, can be given to the patients. Thus, a successful targeting with high amounts of 10B in the tumour cells gives a significant dose enhancement when applied in fast neutron therapy but it is then more reasonable to treat the patient directly with thermal or epithermal neutrons since the average dose enhancement in the latter case is about a hundred times higher and curable doses might be obtained by the tumour specific capture processes alone.