Desirable physical properties of Radio Pharmaceuticals?

Physical properties are not a finite list we can pick and choose from, although there are several properties that can be applied, such as radiance and density. Radiopharmacology is important in radioactive treatment. The kind of properties I would think would be important would be, here's just two: density and radiance. Obviously if technetium doesn't have a great enough density to withstand the conditions of the body, it will not stay in one place very well, which can make it difficult for tracking (no worries with that metal, though). Radiance is important as a physical property because the level of radiation that is used in the body can be important, because too much radiation can hurt a person, and not enough won't do the job right.