Hallelujah! Butch's biopsy came back clean, no cancer cells at all.
When I first posted about the biopsy, nan16 commented: "I have heard that sometimes when there have been numerous infections, scar tissue eventually builds up and it looks and feels like a tumor, much like an abscess can calcify around it after a long time." She was right on the money.
Butch has had repeated infections since October, and the speculation is that there was an anal-sac rupture that kept the area infected with bacteria. For now, he's on another four-week round of antibiotics, and then we'll see.
The vet said she doesn't think surgery is an option in the near future because the tissue in the affected area is too fragile. I wonder, though, if an anal sac ruptured, won't it keep on causing infections one right after another? We go back next Tuesday to get Butch's stitches out, so I'll ask more questions then.
Thank you all so much for worrying right along with us. Your support made it a lot easier to keep a positive outlook while we waited for news.