Driftwood, - I've had my lymph nodes cleared, I haven't experienced any arm swelling, although I know one or two people who have (on this thread Pink has been having problems, - I hope she doesn't mind me saying, - but I'm sure she can give you helpful info.)
IME (and I go to several breast cancer groups, - support, fitness etc.) it is a minority of people who get lymphoedema. But of course if you are in that minority it is not pleasant to deal with.
Also you have to be careful with your arm on that side, - because there are no nodes, you are much more vulnerable to infection. So gloves for gardening and cleaning, and quick action if any looks like it might be infected.
The mon - fri radiation treatment sounds fairly usual to me, - are you worried by the length of time involved? Most people have 3 or 4 weeks of rads I believe.