Sometimes after treatment part of the lung behind the treatment area can become inflamed, causing a dry cough or shortness of breath and If you have treatment to the area around your collarbone, you may develop a sore throat or discomfort when swallowing,this could cause extra mucus to develop; again this usually heals by itself over time
if you are worried talk to your radiographer, oncologist or breast care nurse.
I have heard of other people with the same issue...and no problems..stopped after a shorttime
I hope this helps a little
It might be helpful to give our Helpline a call to talk this over. (Opening times are 9-5 on weekdays and 10-2 on Saturdays. The number is 0808 800 6000)
Very best wishes