For decades cancer specialists have offered ­patients three main therapies: surgery, chemotherapy and radiation. (Some cancer survivors pointedly refer to this harsh trinity as “slash, poison and burn.”) Over the years continual refinements in these admittedly blunt instruments have made the more severe side effects increasingly manageable. At the same time, effectiveness has improved markedly. And new, very targeted drugs (Herceptin and Gleevec) have become available for a few specific cancers. All told, the average five-year survival rate for invasive cancers as a group has risen from 50 percent to 66 percent in the past 30-plus years. In spite of these gains, many cancer survivors will not have a normal life span.