over versus during

"The incidence of cancer during a five-year follow-up" is the correct choice. We use during to talk about something that happens within a particular period of time, to say when something takes place. "During my time in London I visited a lot of interesting places." We use over to talk about something that has been happening continuously up until the present, or will happen continuously in the future. "We've had a lot of problems over the last few months." "We expect a rise in sales over the next few months."