What are worst breast cancer radiation effects?

FIBROSIS: Radiation pneumonitis with fibrosis is the most serious. Also radiation esophagitis and scarring of the skin. Most resolve to some degree with time and steroids.

Fatigue, skin rxn: During treatment: skin redness, fatigue. Late complications such as damage to the heart or arm swelling are far less common and can occur years after treatment.

Skin burns: Radiation to the breasts can cause skin burns which are uncomfortable, causing redness and sometimes ulceration and oozing from the skin. There are also occasional serious organ injuries to the lungs and to the heart. But a good radiation oncologist can minimize all these effects.