Non-malignant lump in cat

Question
We recently had a small mass removed from the back of our 7 year-old tiger tabby's leg.  We have been distracted by the drama and trauma of Boo pulling out her stitches and reopening her incision, but we thought it was all over until we heard from the vet today.  He said her lump was not malignant, but definitely something to watch, as these types of tumors produce immunoglobulin, which, unchecked, would ultimately cause kidney problems.  He also said that these kinds of tumors generally occur around a mucous membrane boundary, and that it's unusual to see it in the leg.  He seems fairly unconcerned, just telling us to watch for any new lumps & have them removed promptly.  Can you tell us anything more about this condition?  Is there anything we should be doing to prevent/limit this from happening again?  The first removal was so traumatic for her (and us)--can you make any suggestions that would make this easier on us all?
I "acquired" Boo when she was a 6-week old barn kitten; I've known her longer than I've known my wife.  I know I'm over-reacting, but she is my baby.  Thanks for your help.
Ben Sims

Answer
Hi Ben!  You are not over-reacting in any way, shape or form.  This IS your baby!  You have every right to be concerned.  Any type of lumps and bumps we find on our furbabies is very scarey!  I am happy to hear that Boo's lump was not malignant.  Any tumor has the potential to grow cancer cells.  It was actually not uncommon for your cat to develop a tumor where she did.  This is refered to as a subcutaneous immunoglobulin, subcutaneous, meaning "under the skin."  Cancerous cells can effect any part of the body and organs, so I'm not quite sure why your veterinarian pinpointed the kidney area.  Cats often develop renal disease in older age, but as of now, I wouldn't see a reason to worry.
There is nothing that you can do to prevent another lump from forming.  It may or may not happen in the future.  The important thing is, to get the lump removed immediately upon discovery.  Since the tumor was subcutaneous, the others, if there will be others, will more than likely be subcutaneous as well.  If cancer does develop, a subcutaneous tumor is much more successfully removed than that of an internal or intramuscular tumor, where tissue, nerves and bone may be involved.  I know it's a bit unnerving when finding something like this, but the important thing is that the tumor has been removed.  
I always recommend doing a weekly at home check up.  Once per week, run your hands all over Boo so that if any lumps do develop, you will know it right away.  The sooner a tumor is removed, the better.
Hang in there, and try to relax.  Boo should be just fine:)
Give her tons of kisses for me!
Hilary