For all practical purposes, there is no significant relationship between a “true” fibroadenoma and breast cancer.
However, there is slight overlap in the mammographic and sonographic appearance of some fibroadenomas and some uncommon forms of breast cancer. For this reason, care should be taken when attempting to make the diagnosis of a breast fibroadenoma without a biopsy.
Usually this means if biopsy is not performed, at a minimum the potential “fibroadenoma” should be followed over an extended period of time (usually a minimum of two years) to be sure it’s not a slow growing breast cancer masquerading as a “fibroadenoma.”
Also, even if a biopsy is performed, there is some overlap between a fibroadenoma and other tumors. Fibroadenomas belong to a class of tumors in the breast known as fibroepithelial lesions. Rarely (exceedingly rare), the tissue obtained from the tumor during the biopsy can be difficult to differentiate from a “relative” of the fibroadenomas in the same class of tumors known as a Phyllodes tumor (of which a small percent can be malignant).
For this reason, even after a biopsy, at least one follow up visit may still be indicated for a fibroadenoma diagnosed by biopsy, but it’s usually just due to an over abundance of caution because no-one wants to miss a sneaky form of breast cancer.