Unfortunately, my only experience with one was with a dachsie who developed one in his chest. By the time it was noticeable as a tumor, it was so invasive, and in such a bad spot- that it was determined to be inoperable. And it did eventually lead to his euthanasia, as it put pressure on his organs.
It didn't seem to affect him at all until the end, which was almost 6 years after we'd first noticed the tumor. He was a happy, otherwise healthy, fairly lumpy dachshund until his final day- really, no sign that it bothered him at all. We think he was 12ish (he was a failed foster who had been found practically feral- so, we really never had a good age).