progressive "death" of cherry tree

We have a fan trained cherry (probably at least 20 years old) which bears delicious fruit. About six years ago, the furthest out side branch developed buds but didn't come into leaf.  The next year, the same thing happened to the next branch in.  This has continued progressively until now over half of the tree is dead.  When I've cut off dead branches, the wood looks sound with no dark spots, although very low down on the trunk there is a large area which looks like a canker (I think).  I give it lots of TLC but can't seem to halt the decline.  Does anyone know what might be the cause and if I can salvage it?  Thanks in advance for your ideas.