Radiation damages the dna in a cell, which kills the cell. This is useful for killing cancer cells.
Cancer is caused by many things including DNA mutations which can take place when cells are exposed to radiation.
Radiotherapy is used because research shows that on terms of the numbers, it does more good for more people (with certain cancers) than harm.
Obviously, chemotherapy and radiotherapy saves lives and kills cancer, that is why we use it. But there are always risks of side effects including mutations from the radiation. If by terrible luck, the mutations scramble the dna code so that the cell starts replicating uncontrolably..... a mass forms, i.e. a tumor, i.e. cancer.
For a perspective, radiotherapy tends to be used to reduce the size of tumors, usually to help in complex surgery and chemotherapy (killing cells with toxins) is then used to kill the cancer cells....if they have specific receptors to the toxins we have developed.
It is important to note we have only had the scientific method for around 150 -200 years and only had the aid of computers for a couple of decades. We have learned and developed much in a very quick, recent time period. Yes we have much to learn but we also can see that we have made huge advances in prolonging life and increasing quality of life. Chemo and radiotherapy use is a tough desicion as healthy cells die too so the patient really suffers. It is all a difficult balance and the doctors make all the desicions based on protocols. Tumour type, location, spread, lymph involvment along with the patients health status all add up to a treatment specific to their needs or in some cases a desicion to not offer treatment. These proticols/guidelines are based on research and change all the time based on new evidence.
snare
Just to highlight the reality of radiation and risk BOTM, mammograms are a good example....
As with all X-rays, having a mammogram exposes you to some radiation, but only a small amount. For every 10,000 women who have regular 3 yearly screening between the ages of 47 and 73, experts estimate that there will be between 3 and 6 extra breast cancers caused by radiation.
However...... The current evidence suggests that breast screening reduced the number of deaths from breast cancer by about 1,300 a year in the UK.