To the OP - all of our older dogs have had a fatty tumor, or two or three or +++. Because of their advanced ages, my policy is to leave them be unless they grow large enough to cause an issue (like one located near the belly/elbow that grows big enough to impair walking, etc). I noticed that these growths became more frequent as my dogs aged, and if I took off every one, they'd have been in surgery continuously, so I only take off maligant growths and fatties that become a problem somehow for other reasons (very rare).
To mimosa - I am so very sorry about the diagnosis for your shi ztu
I've known the daily dread and sadness that a diagnosis like that brings with it, and I know how hard it is to live with that knowledge. But your girl doesn't know what her future will bring (as you do now), so let her daily life go on just as it has in the past without any sign of your grief to worry her - love her up every day, and cherish all her moments with you. I know you are probably doing that already, but just wanted to say don't let the dread overwhelm the joy that you can still find in the time you have left (easy to say, but still very hard to do).