Benign phyllodes tumour

by Jason Wasserman MD PhD FRCPC
May 1, 2024

Background:

A benign phyllodes tumour is a rare type of breast tumour. It is classified as a fibroepithelial tumour because it comprises both “fibrous” and “epithelial” elements. It’s part of a spectrum of tumours known as phyllodes tumours, which can also be malignant (cancerous) or borderline (having features between benign and malignant). Benign phyllodes tumours are non-cancerous and are the most common type among phyllodes tumours. Despite being benign, they can sometimes grow rapidly and become quite large.

What are the symptoms of a benign phyllodes tumour?

Benign phyllodes tumours usually present as a round, painless lump that you or your doctor can feel during an examination of the breasts. These tumours tend to grow quickly, which may cause the skin on top of the tumour to stretch. Most tumours measure several centimetres by the time they are removed.

What causes a benign phyllodes tumour?

The exact causes of benign phyllodes tumours are not well understood. However, they are thought to arise from the periductal stromal cells of the breast, which are part of the connective tissue. Several factors may influence the development of these tumours, including:

• Genetic mutations: Although specific genes have not been definitively linked to phyllodes tumours, some genetic changes may contribute to their development.
• Hormonal factors: The influence of hormones, particularly estrogen, may play a role, as these tumours occur predominantly in women and can exhibit changes in size related to the menstrual cycle.

Your pathology report for benign phyllodes tumour:

Microscopic features

A benign phyllodes tumour starts from the cells normally found in the breast. The tumour comprises a mixture of fibroblasts surrounded by a type of connective tissue called stroma and epithelial cells, which form large ducts. When examined under the microscope, the tumour often grows in a pattern that resembles the leaves on a tree. This pattern can be helpful for pathologists when examining only a small tissue sample. The name “phyllodes” comes from the Greek meaning ‘leaf-like’.

How is this diagnosis made?

Before diagnosing a benign phyllodes tumour, your pathologist will carefully examine the tumour for the following microscopic features: stromal cellularity, stromal overgrowth, cellular atypia, mitotic activity, and growth pattern. These features are described in greater detail in the sections below.

Stromal cellularity

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 is a term pathologists use to describe the number of fibroblasts within the stroma of the tumour. In a benign phyllodes tumour, the stromal cellularity is typically low.

Stromal overgrowth

Stromal overgrowth is a term pathologists use to describe parts of a tumour that are mostly or entirely made up of stromal tissue with very few epithelial cells. Benign phyllodes tumours should not contain areas of stromal overgrowth.

Atypia 

Pathologists use the word atypia to describe cells that look different from normal, healthy cells in the same location. They look at the size, shape, and colour of the cells to decide if they are atypical. The cells in a benign phyllodes tumour are typically very similar to those found in normal, healthy breast tissue and, as a result, are usually described as showing no or only mild cellular atypia.

Mitotic activity

Cells divide 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. The number of dividing cells is an important feature for the pathologist to count when determining the type of phyllodes tumour. For this reason, many pathology reports will include the number of mitotic figures seen in the tissue sample. The count is usually expressed as the number of mitotic figures seen in 10 high-power (high magnification) microscopic fields examined. Benign phyllodes tumours typically have less than 5 mitotic figures per 10 high-powered fields.

Pattern of growth

The pattern of growth is a term pathologists use to describe the way the tumour grows into the surrounding normal breast tissue. It is an important feature. However, it can often only be assessed after the entire tumour has been removed. In a benign phyllodes tumour, the growth pattern is often described as well-circumscribed which means there is a clear border between the tumour and the surrounding normal breast tissue.

Margins

In pathology, a surgical margin is the edge of a tissue that is cut when removing a tumour from the body. The margins described in a pathology report are very important because they tell you if the entire tumour was removed or if some of the tumour was left behind. The margin status will determine what (if any) additional treatment you may require.

Most pathology reports only describe margins after a surgical procedure called an excision or resection has been performed to remove the entire tumour. For this reason, margins are not usually described after a procedure called a biopsy is performed to remove only part of the tumour. The number of margins described in a pathology report depends on the types of tissues removed and the tumour’s location. The size of the margin (the amount of normal tissue between the tumour and the cut edge) depends on the type of tumour being removed and the location of the tumour. For phyllodes tumours, a surgical resection margin of at least 1 cm is recommended to reduce the chance that the tumour will regrow after surgery.

Pathologists carefully examine the margins to look for tumour cells at the cut edge of the tissue. If tumour cells are seen at the cut edge of the tissue, the margin will be described as positive. If no tumour cells are seen at the cut edge of the tissue, a margin will be described as negative. Even if all of the margins are negative, some pathology reports will also measure the closest tumour cells to the cut edge of the tissue.

About this article

Doctors wrote this article to help you read and understand your pathology report. Contact us with any questions about this article or your pathology report. Read this article for a more general introduction to the parts of a typical pathology report.

Other helpful resources

Atlas of Pathology