Dear Atticus222, Although breast cancer is extremely rare in such a young person it can occur. Likely the biopsy is an attempt to err on the side of caution.
A round smooth lump in a 15 year-old is almost certainly a fibroadenoma. What to do to confirm it depends on how worried people are; it's quite safe in most circumstances just to keep an eye on it with rechecks over time, without doing any sort of biopsy. But if there's enough concern, then a needle sample is pretty simple. I assume it's a feelable lump; in which case, depending on the resources available for analysis, the easiest is to use a fine needle, in the office, takes about 10 seconds. Some young ladies, for various reasons, want fibroadenomas completely removed, which is ok, too, but not usually required other than for peace of mind. P.S. just for the record: "cyst" means fluid, not solid. Cysts are nearly unheard of at her age. So calling it a cyst that's solid is not exactly correct.
Well, maybe they didn't call it a cyst then. They pointed to an area that they said was, "full of debree?" But it was surrounded by a cystic area that was fluid filled. I think that they want a sample of both areas. Have you ever seen anything like that?
Thanks
Atticus222