In laymans terms it looks as if his disease is in the jaw bone an has spread quite extensively into the surrounding muscles and tissue.It makes sense that the tissue and muscle invasion will need to be treated with radiotherapy as they dont seem to have a clear margin of healthy tissue to measure against.The word fossa indicates a hole or space and infratemporal is the indentation at the side. of the hinge of the jaw. So my understanding would be invasive carcinoma of the mandible.The affected areas are to do with jaw clenching and chewing and the pterygoid bit is at the junction of the jaw bone just below the ear.I am not a Doctor but i am a registered nurse and this is only my limited view i am sure your Doctors will be able to explain it in full when you see them.