Is a fibroadenoma fatal if untreated?

On the contrary!: Fibroadenomas are the most common benign breast tumor. Excision is not necessary unless there are features that would suggest a rare variant of fibroadenomas, called phyllodes tumors. Despite this, some women will still opt for excision due to the size of the tumor and/or fear surrounding constantly feeling a lump in one's breast. If reassurance fails, the surgery itself is quite minimal.

No : Fibroadenomas are noncancerous (benign) breast tumors. They are very common.