If I had one, and she needed breast radiation, I'd tell her.
That bench/table you'll lie on is not a sofa. It is hard. Don't wear any pants with lumpy back pockets. And skip the cowgirl belt with leather trim and heavy studs.
You'll be taking off all clothes above the waist every day, so wear something that buttons down the front, or something that you can take off gently and easily.
Since you're having external beam radiation, get the ointment at the start! You may never itch. But don't wait to find out at 3 a.m. after a few weeks.
This whole DCIS process can be overwhelming. I went home with the hospital's locker key twice and did an assortment of other dumb things.
ASK questions. Do not be shy.
At first there's a "simulation day" when they decide the exact positions for your radiation, They need to make three tiny permanent marks to help place the radiation. If you don't like them, you won't like getting a rose put on your left hip.
You'll be lying with your arms raised above breast level, on something firm. Since you (my imaginary sister) have tendinitis and arthritis, you may be "uncomfortable." This is hospital code for "your arms, shoulder, and neck may hurt a lot on some days." For instance, on the simulation day and on the first day of the Boost (when they concentrate on one area.) Those days you'll be in position longer. Take Tylenol or ask your doctor what you can take for pain and still drive home without endangering deer, road signs, or motorcycles.
Read the directions about staying out of the sun (a tanning booth is sun), getting plenty of liquids, and getting as much rest as you need. This is no time to change the oil in your car or attempt to bathe Fang.
My personal advice: do not go home on the first day without an emergency after-hours number for your radiation oncologist or the cancer center. If there are special phone instructions, like for a pager, get them to write down the special instructions. No, I don't think you'll need it. Get it anyway.
Buy a very soft "un-bra" to wear to bed. Burn your underwires. Really.
Stay away from people who have colds and whatever.
Treat yourself very nicely.