Question: If Radiation thyerapy is used as a way to help kill off cancer, then why are people who come into contact with radiation in their job (astronaut etc) more likely to develop cancer when they are older?

Leo Garcia answered on 24 Jun 2010:

You are completely right – radiation both kills cancer cells, and can cause cancer to occur in healthy cells. This is because the radiation interacts with DNA that lives within the nuclei of cells. We hope, in radiotherapy, that the DNA of cancerous cells will be damaged beyond repair, killing the cell. However, in other circumstances, the DNA could just be made to mutate – and this in turn could cause cancer.

One of the tricks of radiotherapy is the shape the treatment beam – in a process will call ‘conformal radiotherapy’. Here, pictures from some form of medical imaging, perhaps a CT scan, will be looked at by a medical physicist who will trace the outline of the tumour out in 3 dimensions. This is then fed into the treatment machine computer which figures out how to shape the beam, and from what directions to fire it at the tumour, so that the dose of radiation to healthy tissue is minimised, and the dose to the tumour is maximised.

In terms of people who work with radiation, yes, the exposure you will receive will increase if you are working around radioactive substances or ionising radiation (like a radiologist or astronaut). Fortunately, strict controls are put on radiation exposure in the workplace, and the wellbeing of employees is always at the forefront of any jobs of this kind.

I remember coming home from lectures once, and I told my girlfriend that medical physicists have a higher annual exposure to radiation than other professions – she was a bit concerned, but hopefully she believes me that it makes no practical difference to my health!