Now onto the overall shape (S) of a lesion or entity. First it is important to know that not all lesions/entities will have a definitive shape. You can state that there is no identifiable shape to the lesion/entity.
There are many different shapes in the world, but only a few that are commonly seen on radiographs. These include round, circular, ovoid, linear, and scalloped. The round/circular/ovoid are very similar and which one you use will be based on what you can see on the radiograph. Linear is just like what it says, a linear or straight lesion/entity. Scalloped does not refer to the entire shape of the lesion/entity but the portions of a lesion/entity in relation to the roots. Scalloped or scalloping is when the lesion goes up between the roots and around the apices without displacing the teeth. Below are some examples to help familiarize yourself with these descriptions. (click image to enlarge)
Remember the best way to get better and become more comfortable with describing is to practice on more radiographs.