Radiation therapy uses ionizing radiation to treat cancer i.e. to destroy cancerous cells. The aim of radiation therapy is to cause damage to the cancerous cells whilst minimizing the risk to surrounding healthy tissue. The damage inflicted by radiation therapy causes the cancerous cells to stop reproducing and thus the tumour shrinks. Unfortunately, healthy cells can also be damaged by the radiation. This lecture briefly describe why does the amount of radiation given to the patient have to be accurately calculated.