The thread on this is pretty good, and both Sukie and Stephanie have
very valid points.  The terminology of neoplasms can be confusing, and
I'm sure even vets can be confused from time to time.
 
The proper term for what you are talking about is neoplasm (or literally
"new growth).  For one reason or another, a clone of cells begins to grow
uninhibited by normal substances or mechanisms.  The word "tumor" may be
technically correct, but it is actually non- specific - tumor is Latin for
a swelling, but it could also be an abscess, or anything else that causes
swelling - so I try to stay away from it.
 
Now this neoplasm may be benign - without the ability for cells to detach
into the bloodstream or lymph, move to another tissue, and set up shop -
or malignant (where they can do this.) The process of microscopic piece of
a tumor moving to another organ is called metastasis.
 
Metastasis is the hallmark of malignancy.  The worst tumors have the
propensity to go anywhere and start growing (like lymphoma).  However, we
can recognize malignant tumors even before they metastasize, often by
characteristic features seen under the microscope.
 
Adrenal carcinomas (malignant adrenal tumors) are interesting neoplasms.
Although they possess the ability to metastasize, only a small number do,
and usually only late in the course of disease.  It is likely that they do
metastasize a lot, but have trouble gaining a foothold in distant tissues,
so it takes a long time and many attempts if they ever truly metastasize.
 
Insulinomas are generally not malignant tumors as they only very rarely
metastasize.  The presence of multiple tumors in the same organ over
time is not metastasis.  We do not understand the mechanism behind the
generation of these tumors, and when we do surgery to remove them, we
really are only treating the end point of this process, without address
the cause.  Thus it is really no surprise that the rate of recurrence is
about 40% within 10 months.
 
With kindest regards,
Bruce Williams, DVM
FHL:  http://groups.yahoo.com/group/Ferret-Health-list
[Posted in FML issue 3516]