Radiation sickness, also known as acute radiation syndrome or radiation poisoning, is damage to the body caused by receiving a large amount of radioactive material in a short period of time (acute). The amount of radiation absorbed by the body will determine the level of damage to the body.

Ionizing radiation originates from external sources such as gamma rays, X-rays or from radioactive isotopes introduced into the body for the purpose of treating diseases. Radiation exposure occurs when the concentration of ionizing radiation entering the body exceeds the safe level, causing serious health effects and leading to other dangerous diseases.

Radioactive iodine treatment for cancer is internal radiation therapy that uses a radioactive form of iodine. Radioactive iodine circulates throughout the body in the patient's blood. Cancer cells have the characteristic of absorbing iodine from anywhere in the body, so they will be destroyed.