Newsflash: a topic is different from a thesis. A topic is the subject of your essay—The Great Gatsby, World War II, healthcare reform, whatever. An essay, however, always says something—makes a claim or puts forth an opinion—about a topic, which is where the thesis comes in. Writers often go wrong by describing a topic rather than making a point about it in their thesis. There. Now you should be able to write about how The Great Gatsby affected healthcare reform during World War II. Or something more to the point.