If the radiation only kills the primary tumor, you are still at grave risk if cancer remains in the lymph nodes as it can spread throughout the body where it is much harder or impossible to treat. For example, my husband's treatment included the most radiation to the primary and positive node, somewhat less radiation to the other nodes on that side, and still less to the nodes on the other side of the neck. The radiation planning will include different radiation amounts to different areas on your neck. The radiation planning takes a lot of math/physics understanding, and it totally over my head aside from the most general terms - and I've had graduate level math course. Make sure your RO seems to be smart and experienced.
Maria