Six new proton-beam centers are set to start delivering state-of-the-art radiation to cancer patients around the country by year’s end. Ten more are expected by 2018, bringing the U.S. total to 30—many the size of a football field and costing between $100 million and $200 million to build. The projects, long in the works, will enter an uncertain market. Proton-beam therapy, a highly precise form of radiation, has been dogged by a lack of evidence that it is better than traditional radiation despite costing significantly more… Some hospitals have turned to private donations, rather than private equity, to finance proton operations. Next month, the Mayo Clinic in Rochester, Minn., plans to start treating patients at its $180 million proton center, one of two built with the help of a $100 million gift.