SuperWanda - don't blame this on yourself. Mass Cell Tumour does not seem to be that familiar with vets. Biopsy after biopsy always came back unconclusive. If unconclusive then why worry right??
Even the removal is not a guarantee that it will come out somewhere else. They have a tendancy to spread very quickly, just keep your eye on that.
So sorry that you are watching this - it is horrible I know only too well.:sad: