Several authors have discussed the effects of inhomogeneities upon the dose distributions due to electrons (1-5), and, as indicated in Part I of this paper, the effects can be appreciable. A number of attempts have been made to take these inhomogeneities into account in treatment planning. The situation is quite complicated, however, and, although the absorption of electrons is primarily determined by the number of electrons per gram which is almost constant for all materials except hydrogen, scattering depends strongly upon atomic number. Since the dose at any point is determined by the dose from the primary beam plus the dose from the scattered electrons, the absorbed dose will be different in materials of different density. The dose distribution is also dependent upon the range of the electrons which is inversely proportional to the density of the irradiated body when Z/A is virtually constant and the atomic numbers do not radically differ. Relative dose distributions are normally measured in water, whose density is closely equivalent to that of muscle tissue, but from the findings in Part I it would seem essential to determine the dose distributions when different types of body tissue and air cavities are involved. Several situations present themselves, as, for example, when the dose is required in muscle tissue beyond an inhomogeneity such as bone, or when the dose is required in the inhomogeneity itself. These cases will now be considered. Bone The effect of bone on the dose distribution within the bone or at the interface and closely surrounding areas may be quite pronounced, as indicated by Breitling and Vogel (2), due to the change in the scatter contribution. This is difficult to determine. It is often required, however, to know the dose in underlying tissue that is shielded by bone, at distances beyond the immediate effect of this scattered contribution. The alteration in the dose distribution is then due to the change in the range or the attenuation of the electron beam. As indicated in the introduction, this is dependent upon the density of the material in the form which is the number of electrons per cubic centimeter, and, as mentioned in Part I, the density of bone can vary quite considerably. For example, if the density of the ramus of the mandible is taken as 1.85 g/cc, then the ratio of electrons/co for mandible to muscle is 1.65. That is, a given thickness of mandible should show the same absorption as 1.65 times its thickness of muscle tissue. This has been shown to be the case with the in vivo dosimeters measuring the dose a few centimeters behind the mandible (6). As reported in the Workin-Progress section last year (7), however, sternum appears to have no effect upon the central axis depth dose or, in some cases, less effect than water.