Rob Leighton’s colon cancer had persisted through a barrage of treatments when his doctor suggested a new test. The Westbrook man had already undergone the standard treatments following his diagnosis in March 2016: surgery, radiation, chemotherapy and even immunotherapy, a newer generation of treatment that boosts the immune system’s ability to fight cancer cells. Still, the tumors continued to grow, and had spread from Leighton’s colon to his liver and lungs. “I was pretty much running out of options,” Leighton said. His oncologist asked if he wanted to try another approach.