This article was last reviewed on November 14, 2019 by Livia Florianova, MD FRCPC
The normal breast
Adult breast tissue is made up of small structures called glands which are organized into groups called lobules. Under certain conditions, these glands can produce milk, which is transported to the nipple by a series of small channels called ducts.
The inside of both glands and ducts is lined by specialized cells called epithelial cells which form a barrier called the epithelium. The tissue surrounding glands and ducts is called stroma and contains long, thin cells called fibroblasts.
What is a fibroadenoma?
A fibroadenoma is a non-cancerous breast tumour. It is the most common breast tumour in women under 40 years old. Fibroadenoma is made up of epithelial cells and fibroblasts.
Fibroadenomas are usually round tumours that feel firm to the touch. Their size may change depending on the body’s hormonal status (for example pregnancy or menopause).
Intracanalicular and pericanalicular patterns of growth
The cells in a fibroadenoma can grow in two patterns called intracanalicular and pericanalicular. These patterns of growth can only be seen after the tissue sample is examined under the microscope. Both patterns of growth are non-cancerous.
How do pathologists make this diagnosis?
The diagnosis of fibroadenoma can be made after a small sample of tissue is removed in a procedure called a biopsy. Some reports may use the term ‘fibroepithelial lesion’ to describe the changes seen on the biopsy. This is a descriptive diagnosis that pathologists use when they have not seen enough of the tumour to make a full diagnosis.
The diagnosis of fibroadenoma can also be made after the entire tumour is removed surgically and sent to a pathologist for examination.
A fibroadenoma will not occur again if removed completely, however new fibroadenomas may develop in the same breast on in the breast on the opposite side of the body.
This is the size of the tumour measured in millimeters. Tumour size will only be described in your report after the entire tumour has been removed. The tumour is usually measured in three dimensions but only the largest dimension is described in your report. For example, if the tumour measures 40 mm by 20 mm by 15 mm, your report will describe the tumour as being 40 mm.
Stroma is the connective tissue that surrounds the ducts and glands in the breast. Most of the stroma is made up of cells called fibroblasts.
Stromal cellularity describes the number of fibroblasts in the fibroadenoma. The cellularity can be variable, and may be slightly increased in younger women. On the other end of the spectrum, the stroma of a fibroadenoma may decrease in cellularity over time, in which case it may be called “sclerosed” or “hyalinized”.
Cells divide in order to create new cells. The process of creating a new cell is called mitosis and a cell that is dividing is called a mitotic figure.
A small number of dividing cells (mitoses) may be seen in a fibroadenoma. The number of mitotic figures may be included in your pathology report.
Why is this important? It is common to find a small number of dividing cells in a fibroadenoma. Dividing cells do not change the diagnosis.
Atypia is a word pathologists use to describe cells that look different from normal, healthy cells in that same location. Most fibroadenomas do not have any atypical cells. When atypical cells are seen in a fibroadenoma, they are usually fibroblasts in the stroma. However, the epithelial cells may also occasionally be atypical.
Why is this important? It is common to find a small number of atypical cells in a fibroadenoma. Atypical cells do not change the diagnosis.
Other changes seen in a fibroadenoma
There are many other non-cancerous changes that can take place inside a fibroadenoma. These changes can only be seen when the tissue is examined under the microscope.
Non-cancerous changes may include:
Why is this important? These non-cancerous changes are common. They are not associated with an increased risk of developing cancer in the future.
A margin is the normal tissue that surrounds a tumour and is removed with the tumour at the time of surgery. A margin is considered ‘positive’ when the tumour cells are seen at the cut edge of the tissue.
Since fibroadenoma is a non-cancerous tumor, the report may simply state that the tumour was “completely excised”, or that margins are negative.
Margins are only described in your report after the entire tumour has been removed.