1.   Radiation dose is measured in sievert. We are exposed to background radiation all the time. When radiation reaches the body, the body is said to be irradiated. When radiation gets inside the body, the body is said to be contaminated. High radiation dose kills cells. Low radiation dose damages the DNA of cells. The risk of cancer increases if the exposure to ionising radiation increases.    2.  When gamma radiation strikes a living cell, it is either killed or becomes cancerours.    3.  Cancer treatment sometimes involves the use of gamma radiation to kill cancerous cells.         The risk to the healthy cells is reduced by making sure that they receive a smaller dose of         gamma radiation.