My 4yo guinea-pig developed a massive lump (in the other thread) , took her to the vet expecting it to be an abcess and to have it lanced and antibiotics.
Not how it turned out 
It was lanced but not pus , it was a growth. . As it was in her neck, all the blood vessels (jugular and carotid) and given she was middle-aged , I had to think "Is surgery the best option?" and hand on heart the answer is "No it isn't"
I have no bloody clue how this lump got so big so quick. The vet said it may have bled into the mass making it suddenly big?
She was eating and poohing right to the end , so I know she didn't suffer . She went peacefully at the Vet with her long term cage mate at her side.
These little animals burrow their way into your heart and when they leave you they rip a bit of it out. That ripped space is best filled with another guinea-pig .
We have GP6 and GP7 still who are now a bit "Eh?"
Goodbye little piggie . The well trodden path to The Rainbow Bridge welcomes a new little guest .