Headaches are not usually a side effect from breast radiotherapy but people can be unique in the side effects they experience.
Headaches can be caused by a number of different which you can see on the NHS website here. Most of these causes will be harmless and will settle after a little while .Taking a mild painkiller such as paracetamol can help.
Hopefully some of our forum members will come along and share their experiences with you, as support like this can be invaluable. But even if they do get in touch, I'm not sure that their experience will shed much in the way of light about what is going on with you.
Give us a ring if you would like to talk things over. The number to call is Freephone 0808 800 4040 and the lines are open from 9am till 5pm Monday to Friday.
