This might not have a real answer, but here goes:
How do doctors determine whether a subsequent diagnosis of cancer is a recurrence of the previous tumor (i.e., at least one cancerous cell survived the previous treatment), or a new occurrence in which an errant cell sprung up independently of the previous tumor?
Well, sometimes it can be obvious. For example, if the second cancer is clearly located in a position typical for a cancer for of that organ (e.g. bronchus i.e. breathing tube, or prostate, or breast) and/or if, under the microscope, the cancerous cells have an appearance distinctly different from that of the first cancer.
Sometimes, it much more difficult. This is more likely to be the case when the cancer has clearly spread outside of the organ where it started (e.g. into a lymph node or the liver). Still, it may be possible by studying under the microscope the cancer cell’s shape and other features. Sometimes the original cancer may have contained certain “inclusions” or granules. If these are present in the new cancer, it probably means they’re one and the same. Further, with the advent of chromosomal and genetic testing, it may be possible to prove that the first and second cancers are the same by virtue of both of them possessing identical genetic make-ups.
Sometimes, the first tumor is such that a late recurrence is essentially impossible. In other words, the first cancer may have been of the type where even if a single cell survived, you’d be dead in five years (i.e. because of a rapid doubling time). An example of this might be a so-called “small cell lung cancer”. So, if the second cancer was detected, say, six years, later, it must be a new one.
Bottom line, though, is that it may not always be possible to distinguish between the two.
By the way, I should have mentioned that it is not at all uncommon to be unable to determine where a first cancer has started. This, of course, can lead to some therapeutic and prognostic uncertainties. Further, if another cancer is discovered, say, four, years after that first cancer has been detected, it may well be unclear whether it is truly a new cancer or whether it’s the original cancer now appearing in a different form.
Here is a link to this entity known as ‘cancer of an unknown primary’.
At the risk of monopolizing this thread, I just realized that I had meant to mention one other thing in my first post, above. In particular, in addition to clues provided by the site of the second cancer, the morphology (size, shape, etc.) of its cells, the presence of various inclusions and granules in its cells, and genetic profiling, there is another major way of identifying and characterizing it (and, more generally, characterizing many cancer cells). I am referring to immunocytochemistry. This is a technique where the cancer cells are exposed to antibodies (with the latter often labelled or tagged in such a way as to make them and any parts of cells or cells that they bind to, quite obviously visible through the microscope). In this way, depending on the antibodies used, one can often get a pretty good idea from which organ or tissue the cancer started.
Immunocytochemical and allied methods are an invaluable tool to identify and characterize cancer cells. And, again, they may be of particular utility in helping to determine whether a second cancer appearing in a person with an earlier cancer, represents a distinct new tumor or, rather, recurrence of the original malignancy.
We do this every now and then in our lab for B and T cell lymphomas. We can prove pretty clearly whether or not two samples have the same clonal populations. It’s loads of fun.