Peter Mac Radiotherapy Experience

I'll be starting radiation therapy in June, I'm having the tattoo version. What exactly happens when they are giving you the treatment, do you have to hold your breath for long? I didn't ask that question at my initial meeting, and I can't remember if they mentioned it. I'll be getting radiation on my right breast, under arm, lymph nodes inside right breast/chest area and clavicle area as the lump was inside right breast. I had 1 lymph node involved,  the rest taken were clear, so they decided against more surgery in favour of more extensive radiation therapy. I'm not overly concerned about the procedure, but interested to know what happens.