How quickly do fibroadenomas grow? Would an MRI clearly show it is a fibroadenoma? And how do they appear on MRI?

Usually slowly. In general, most fibroadenomas grow slowly. However, there is a condition called a Phyllodes tumor that looks like a regular fibroadenoma, but grows very quickly. Sometimes it can turn into a special type of cancer, so I generally remove fibroadenomas. An MRI can suggest a fibroadenoma, but the only way to know for sure is to get a tissue sample--a biopsy.