  We present calculations and simulations on the role of the
p+$^{11}$B$\to$3$\alpha$ reaction in proton therapy. This reaction has been
recently suggested to be responsible for a decrease in the survival probability
of tumor cells, when they are irradiated with low-energy protons. However, at
the concentration levels typical of the proposed boron carrier (sodium
borocaptate, Na$_{2}$B$_{12}$H$_{11}$SH, in short BSH), i.e. less than 100 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.
