Physicist here. In a scientific program I would most likely not use radius. I definitively would not use dR (type should be clear, no need to use Hungarian notation which I find to be an abomination) and would most likely use either R or r, the choice depending on what symbol was used in the equation(s) written in standard form. I probably would include in a comment the relevant equations using TeX notation and a reference to a scientific article or book from where it was taken.