My former best pal, died Oct 12, 2010 from VAS (Vaccination-Associated Fibrosarcomas). Basically, tumor caused by excess vaccinations and shots the vet always like to call you for every year (or multiple times per year). He was VERY healthy for his age (12) and I thought this lump on his shoulder was odd when I originally felt it. Figures it only started developing a few weeks after his last shot. He was a mixed breed of Abbyssinian/Siamese. Very smart, he knew his name when called (and other people's names too), knew the names for certain things, and he knew how to do tricks on command.
And this was 2 weeks before he was put down. Notice the tumor growth on his right shoulder/arm, and how he can't even put pressure on that arm. Has to keep the arm lifted...
Here is a video from a few years ago I took of him doing tricks on command (I tell him to sit down and then roll over):