What is the fine-grained topic of the following text: RETAIN was a two-year randomized, controlled phase 3b study that included 372 patients with diabetic macular edema randomized to one of three treatment groups: ranibizumab administered according to a treat-and-extend protocol plus laser photocoagulation on day one, ranibizumab administered by treat-and-extend, and ranibizumab dosed prn. Only return the topic.
RETAIN phase 3b study on different diabetic macular edema treatments