Radiation After Breast Cancer Surgery & Chemotherapy: Safety & Timing

Question
My sister has breast cancer at an early stage and is getting her final chemo treatment this week. She has been told that her radiation will start 4 weeks later. However, we have paid for a cruise that is between Feb 10 and Feb 20. She wants to go on the cruise but is it safe to delay the start of her radiation for 3 weeks?

Answer
I would prefer to let her have her radiation therapy as soon as possible and in my personal opinion her therapy has a higher priority than a cruise! However I can not say in honesty that her therapy will fail with certainty if it is delayed a few more weeks. But I do not like it! However the choice is hers!