A ferrous sulphate gel with a proper composition to thermalise epithermal neutrons with tissue equivalence with brain tissue gives the possibility of making phantoms which act as a continuous dosimeter for the gamma radiation, with the possibility of 3-D dose determination. If in the phantom a volume of gel containing B-10 (in the amount typical for BNCT) is set, information on the absorbed dose in the tumour site may also be drawn.