three field or four field radiation?

My understanding is that it means the no of different angles which the equipment uses to treat the breast/lymph nodes. Each angle directs the radiation to an area of the breast i.e. a field. This will vary according to whether the lymph nodes need treating and also to the shape of the breast since they try not to overlap the fields. E.G. I am having 4 field radiation and have a dose from each side of the breast which zaps the lymph nodes under the arm too, one to the lymph nodes near the collar bone from the front and one to the same area from the back.

 Hope this helps.