By Richard Knox In his full-bore battle against brain cancer, Sen. Edward M. Kennedy resorted to treatment many consider experimental -- proton beam radiation therapy. Sen. Kennedy returns to Senate last year after treatment for brain cancer. Medicare pays for it. But his death leaves open a slew of questions about the costly treatment, which delivers high doses of radiation to tumor cells while largely sparing healthy tissue from damage. Did it do him any good? Should Medicare (or private insurers) pay for the unproven treatment? And most politically fraught, should Kennedy's legacy issue -- universal health care -- include...