To understand how radiotherapy works, it is helpful to know about the phases that all cells in the body go through to grow and divide to form new cells.

• In the resting phase, the cell hasn't started to divide.

• Then, the cell is stimulated to reproduce, perhaps because other cells have been damaged or are worn out and need replacing.

• The cell makes a copy of its genetic code (its DNA) so that there will be one copy available for each new cell when the original cell divides.

• The cell forms other essential proteins that it needs to function.

• The cell splits into two new cells. This last phase takes only half an hour to an hour.

Radiation works best at killing cells that are dividing, particularly if they are dividing rapidly. It isn't as good at killing cells that are resting or that divide slowly.

To kill as many cancer cells as possible, doses of radiation may be given five days a week for four weeks to six weeks. The doses are given in this way because not all of the cancer cells will be dividing at the same time. So to destroy as many cancer cells as possible you need to be treated for several days at a time. Because normal cells rest for longer before dividing, they are less likely to be destroyed by radiation.

Doctors have also tried giving higher dose radiotherapy for a shorter period of 3 weeks. Studies suggest this works just as well. ^[52]

The dose of radiation that you need will be carefully worked out by your doctor.

When you have radiotherapy, you will probably be treated with a linear accelerator machine that focuses high-energy x-rays onto your breast. Your breast will be marked to show where the treatment should be focused, and you may have to lie still for a few minutes. You will probably be given radiotherapy eight weeks to 12 weeks after surgery.

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