At least 8 classes of compounds are being evaluated in various laboratories around the world as possible vehicles for the transport of boron to tumor for neutron capture therapy (NCT). A parameter of major importance is the minimum concentration of boron needed in tumor in order to produce improved results in cancer therapy. Calculations are made here of the minimum boron content in tumor necessary for NCT. These estimations are obtained for various neutron beams, on the basis of therapeutic gain produced by the effective dose (absorbed dose X relative biological effect). The effects of repair are considered, in anticipation of having boronated bio-molecules with selective and long-term binding to tumor cells, thus allowing protracted irradiations. Pure epithermal neutron beams (free of significant fast neutron and gamma contamination) are found to offer major advantages, particularly when the effects of repair are included. The various boron compounds being investigated for NCT are evaluated on the basis of necessary minimum boron content in tumor.