Radiation therapy is usually applied to the cancerous tumor due to its ability to regulate cell growth. Ionizing radiation works by damaging the DNA of cancerous tissue resulting in cellular death. To spare normal tissues (such as skin or organs which radiation must undergo to treat the tumor), shaped radiation beams are aimed from several angles of exposure to intersect at the tumor, providing a way larger absorbed dose there than within the surrounding healthy tissue. Besides the tumor itself, the radiation fields can also include the draining lymph nodes if they're clinically or radiologically involved the tumor, or if there's thought to be a risk of subclinical malignant spread. It is necessary to incorporate a margin of normal tissue round the tumor to permit for uncertainties in daily set-up and internal tumor motion.