I, too, am sorry to hear that. I also agree with those advocating getting a second opinion. As for the tumour itself, it all depends on the type and growth rate as to how quickly it'll show up on an x-ray. If it wasn't there a month ago, and it is now, then your vet's right and it's pretty aggressive. However, it may be locally agressive, i.e. only in that particular bone, and maybe the adjacent muscle(s), and has not yet spread to other organs (e.g. the liver), in which case an amputation with chemo &/or radiation may be curative. You should also consider further imaging as it might be helpful in making a decision. Good luck and hang in there.