Not a good idea
Of course it's your body and you can do whatever you want! but i would advise that you wait until after chemo and radiation to get a tattoo.
Why? During treatment you have a higher risk for infection, skin dryness/peeling, and reacting to the ink. The bigger risk is probably chemo. If your radiation is aimed at a body part far away from your tattoo, it might be ok, but ask your doctor.