On the (un)effectiveness of proton boron capture in proton therapy

We present calculations and simulations on the role of the p + 11B → 3 α reaction in proton therapy. This reaction has been recently suggested to be responsible for a decrease in the survival probability of tumor cells previously treated with a boron compound, when they are irradiated with low-energy protons. However, at the concentration levels typical of the proposed boron carrier (sodium borocaptate, Na2B12H11SH, in short BSH), i.e. less than 100 parts per million (ppm), both calculations and Monte Carlo simulations suggest that the dose related to this reaction is orders of magnitude lower than the dose delivered by the primary proton beam inside the tissues. These calculations cast some doubts on the claim of an important role played by Proton Boron Capture in enhancing the therapeutic effectiveness of proton therapy, and suggest that other mechanisms should be investigated in order to explain the observed decrease in the survival probability.