Well...not a vet but am a Biology/Molecular Biol/Biomed student...not sure if that helps...

Is it attached or loose?

So...does it move indepentenly and can you put your fingers 'behind' it? Or, is it 'attached' and not able to be moved? THis is the main different between a fatty lump/cyst and a tumour, I believe. Also, does it feel soft and fluid filled/squishy, or is it very firm?

Vet is a GREAT idea. My Siamese had one in the same spot right next to nipple which looked totally harmless and fluid filled - like a cyst - and burst normally like a cyst....but it was breat cancer. Don't want to scare you though.

Just re-read her breed. WOAH she must be huge! Awwwwww!

And swoon swoon swoon at your Bassett!

Sorry...major tragic animal lover :-P