There are many types of cancer that can metastasize to the bones. When
prostate tumor cells migrate to the bones and replicate there, as I
understand it, the prostate cells are taken up in growing bone. These
cells are still prostate cells and can still produce PSA. These are
called "secondary" bone cancers because they don't originate in the bones.
Other kinds of tumor cells, for example breast cancer cells, can also
migrate to the bones. These other secondary bone cancers (i.e., cancer
cells in the bone that did not originate in the bone), do not produce
PSA. As I understand it, primary bone cancers, e.g., osteosarcoma, also
do not produce PSA.
It is only prostate cancer that produces PSA - whether it's in the
prostate, in the bones, or elsewhere in the body.
Alan