My dad is undergoing t/t for largebtype diffuse nhl.my concern is will it come back once the cancer is gone after 6cycles of chemo?

We'll see: Diffuse large b-cell is often curable with chemotherapy -- when i was in med school this was a dream that it might someday be possible, and now it is a dream come true. His oncologist may have discussed his odds. Your best task is to try to stop thinking of the likely outcome, and be as good to dad as you can -- you care, and he's probably really glad to have you as a son. Cherish time together.