You will hold your arm above your head during radiation. This creates a crease/fold of skin in the collarbone/neck area. This area might start feeling "sunburned" after a few weeks of treatment. My skin got a little pinkish and didn't look bad but suddenly one day that area became very raw feeling. I used clear Aloe Vera gel as instructed but using it more often didn't help. The nurse finally gave me a free sample tube of Biafine which helped greatly. The radiation area in general will become dark or slightly tanned looking. The creases between folds of skin are extra sensitive.
Your skin might look tanned or pink compared to the other side. Toward the end it might little tiny moles all over - kind of a freckly appearance. It's pretty funny really. Don't worry - within a month after your last radiation, "the girls" will start looking alike again.
After it's all done with, the irradiated skin will peel just like sunburn does. Mine took a few months to get through this process. I had racing stripes of new skin and old irradiated skin that was darker.
Another side effect is that radiation makes the bones that got irradiated become porous - brittle bones. You'll need to take calcium the rest of your life which most of us should be doing anyway. And you can't do anymore tackle football! No impacts allowed because a bone could break (more easily than before). Seriously - impact sports and car accidents are the things to avoid.