The purpose of this work is to analyze dose distribution inside tissues. To do this, we performed some MCNP simulations using the neutron flux obtained from the Kyoto University Reactor. We have tried to analyze the behavior of neutrons in different types of tissues in relation to their depth. We have found that the value of dose from neutron interaction with 10B depends not only on 10B concentration inside the tissues (a higher concentration produces a higher dose), but also on the tissue density. In fact, tissues with a density considerably different from that of water receive a lower dose. Another dose contribution is given by the presence of 14N inside tissues: this dose contribution is lower compared with the previous one; it is influenced both by the tissue density and the percentage of nitrogen inside the tissue. Finally, the delivered dose decreases very quickly after a depth of about 4 cm, which implies that boron neutron capture therapy is not an effective therapy for the deepest tumors. However, there are some factors that can be taken into account to reach the deepest zone.