Although I love the idea of it, the clinical utility has been contentious. My sense of it is that, like tumor markers, may be relevant to some but not to others. I suspect big numerical swings are more reliably indicative. I believe there is someone on this board who is maintaining stage IV by reintroducing Herceptin when her serum her2 goes up.
Have you had a serum
circulating tumor cell (Veridex etc) test?
Also..if you haven't had a biopsy since the primary, might give you some leverage in getting the serum her2 test or a new biopsy. The onc should be comfortable with the idea that pathology of mets can be different from the primary. For example, if a new biopsy showed ER+ mets, a new avenue of treatment would open up. The mets may have switched to using estrogen due to
previous her2 therapy. If getting a biopsy is an option, you could also push for a
chemosensitivity test...though you may have to pay for that.