Should I change my diet or lifestyle because of radiation therapy?

Depends on cancer. Other than stopping smoking if you have lung or breast cancer, there are no diet or lifestyle changes that will affect your radiation therapy. However if you make lifestyle changes when you have been treated for cancer, then your longterm survival of that cancer is likely to be improved.
Depends. Nutrition for all patients under treatment for cancer is important. Specifics for radiation especially depends on site of treatment - please discuss with your radiation oncologist. A good general resource is the publication by the national cancer institute.