Alternatively, have antibodies tailored to identify and attack any cancerous cells. The same effect as the Nanotech, but it enhances the existing defenses so they can eliminate any cancers before they even get big enough to be noticed by the host. Antibodies can go rogue and attack cells in certain circumstances anyway, simply tinker with them so they can tell which need killing with better accuracy and there'll no more cancer. There will still be cancerous cells forming in the body, but they won't have a chance to grow and become a problem.
Like Steel says, I also think that, given how much research is being done into cancer in the real world, it will probably NOT be a problem in an Sufficiently Advanced culture. I see cancer becoming one of those quaint, distasteful diseases that only primitives die of (like tuberculosis in modern times) by Biotech 5-6.