Benign phyllodes tumour

by Jason Wasserman MD PhD FRCPC
August 16, 2023

What is a benign phyllodes tumour?

A benign phyllodes tumour is a rare non-cancerous type of breast tumour.

Can a benign phyllodes tumour turn into cancer over time?

It is very rare for a benign phyllodes tumour to transform (change) into a malignant (cancerous) tumour over time and only a few cases have ever been reported. However, a benign phyllodes tumour can regrow into the same location if not completely removed.

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?

At the present time, doctors do not know what causes a benign phyllodes tumour to develop.

How is this diagnosis made?

The diagnosis of a benign phyllodes tumour can only be made after the entire tumour has been surgically removed and examined under a microscope by a pathologist. A preliminary or initial diagnosis can also be made after a small tissue sample is removed in a procedure called a biopsy.

What does a benign phyllodes tumour look like under the microscope?

A benign phyllodes tumour starts from the cells normally found in the breast. The tumour is made up of 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’.

What are the microscopic features of a benign phyllodes tumour?

Before making the diagnosis of 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 pattern of growth. 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 made up mostly or entirely of stromal tissue with very few epithelial cells. Benign phyllodes tumours should not contain areas of stromal overgrowth.

Atypia 

Atypia is a word pathologists use to describe cells that look different from normal, healthy cells in that same location. Pathologists 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 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. 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 pattern of growth is often described as well-circumscribed which means there is a clear border between the tumour and the surrounding normal breast tissue.

What is a surgical margin and why is it important?

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 for the purpose of removing the entire tumour. For this reason, margins are not usually described after a procedure called a biopsy is performed for the purpose of removing only part of the tumour. The number of margins described in a pathology report depends on the types of tissues removed and the location of the tumour. 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 provide a measurement of the closest tumour cells to the cut edge of the tissue.