Here's the email exchange.
I went out to the barn on Monday, and Tristan's wrap had come off when he came in from the field, finally. (It was on for 5 weeks!)
There is...nothing at all on his tail. No stitches, no lump, only a vague maybe-sorta outline where the lump was.
So, I guess it's all over with? I'm puzzled but glad, I guess. Weirdo horse.
Her reply. The bolding is mine.
It's our beads! Non-cancerous tumor or not, they shrunk the growing tissue. As an aside, I'm thinking of doing a research project on this. It is very interesting. Glad they are gone and he is well!!
My horse: subject of a research paper, coming soon. If she actually does write it I will absolutely share it.
(For those wondering what happened to the radioactive beads: I, too, am wondering. My best guess is that since they were "bioabsorbable" they did indeed absorb fully and are gone, and that the half-life of the radioactivity was such that there is no concern. I have emailed the vet back to make 100% sure, though. She is SUPER on top of things, so if there were any danger I'm quite confident that she would have followed up.)
At least he is cute.