Boron implantation into preamorphized Si, followed by low temperature solid phase epitaxial (SPE) regrowth produces high activation combined with low diffusion. However, in the presence of high B concentrations, the activation obtained after the SPE regrowth only can reach concentrations in the order of a few times 1020 cm−3, and even more deactivation occurs during additional annealing. We have analyzed the role of the Si interstitials injected from the end of range (EOR) damage in B deactivation and reactivation by atomistic simulations. We have shown that the B cluster evolution can be clearly correlated to the evolution of Si interstitial defects at EOR. This is also compatible with B cluster stabilization in the presence of excess Si interstitials, observed in oxidation experiments.