Hi diamond, yes pneumonia can be a side effect of breast radiation depending on the type of radiation you receive. I had 15 sessions with heart sparring breathing but was warned that the upper curved area of my L lung could develop scar tissue and pneumonia could be a side effect. It is not bacterial pneumonia that develops but a different type. Coughing is a symptom but not just the type of cough that comes from a dry throat. Contact your radiologist about your symptoms and or get a second opinion, my kind psychologist told me at the outset of all this always get a second opinion no matter what the issue is if you have concerns. Best to you.