My 8 year old rescue cocker had an aggressive tumour there, from me taking him to the vet when I saw it, it was monitored for a week, and grew exponentially. He had it removed, a big operation, but sadly he had secondaries in his spine and so we had to let him go. We didn't know about the secondaries til a bit after the op, if I had known I wouldn't have put him through it. Can your vet not do a biopsy to find out what kind it is?
I'm so sorry that this has happened to poor Willow, and sending you hugs .
Lesley