Ulrike - I had a simular reaction when I underwent radiation. The doctor and nurses all told me to make sure I lather up with Keri Original, Jergens with Aloe, or the like type of lotion 3 times a day. I went from being regular skin toned to burnt to a crisp overnight. But once radaition ended (in Aug) my skin bubbled & peeled like a sunburn and now I'm just a tad darker. The lotion really helps. You should likely be checking in with your radiation dr weekly - have him look at the burn and see what he thinks. He may rx some special aloe gel.
Good luck!
Angi