Besides the tumor itself, the target for the radiation may also include the lymph nodes that drain the area. Doctors include the lymph nodes if they are clinically or radiologically involved with tumor, or if there is thought to be a risk of that individual cancer cells will spread. It is impossible to aim the radiation beam exactly because the tumor moves as the patient breathes. So, doctors also expect to include some healthy tissue around the tumor to make sure that the entire tumor is hit by radiation.