Yes! There is fast food in Rome. McDonald's and Burger King restaurants seem to be around almost every corner. This sign for McDonald's with it's slogan "I'm lovin' it." has been expanded to say "I'm lovin' italy."
I am not aware of a Burger King near the Colessium, but this mural seems to say that you can have it the same way as Caesar had it.
You often don't see McDonald's in the historic center because the restaurants blend in with the surrounding architecture and monuments. This picture of the Piazza della Rotundo in front of the Pantheon includes a McDonalds! (It is the ground floor of the peach colored building.)
People are enjoying Quarter Pounders, Big Macs and french fries while looking at the Pantheon.