Use of I‐125 in breast implants may be therapeutically beneficial due to the 40% higher dose to the tumor relative to the normal adipose tissue. The dose distribution in the interface between adipose and nonadipose tissues is studied with mathematical models and radiobiological measurements. The dose transition zone is narrow, about 10μ, which is roughly the diameter of a cell. The dimension of the transition zone is weakly sensitive to the geometry of the interface. In most geometries, cells on either side of the interface receive doses that are significantly different (25%–35%). Radiobiological measurements are consistent with calculated results, providing a check on the theoretical model.