Phyllodes tumour

by Brandon Zelman DO and Jason Wasserman MD PhD FRCPC
March 10, 2022

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What is a phyllodes tumour?

Phyllodes tumour is a rare type of breast tumour. Most phyllodes tumours will behave like non-cancerous tumours. However, a small number (approximately 10%) will behave like cancer. This behaviour includes the ability to spread through the normal tissue in the breast and to other parts of the body.  The entire tumour must be removed surgically before your pathologist can decide if the tumour is a non-cancerous or cancerous type of phyllodes tumour.

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.

How do pathologists make the diagnosis of phyllodes tumour?

The diagnosis of phyllodes tumour can be made after a small tissue sample is removed in a procedure called a biopsy. However, the biopsy may not provide enough tissue for your pathologist to decide if the tumour is benign, borderline, or malignant.

After the tumour has been removed completely, it will be sent to a pathologist who will prepare another pathology report. This report will confirm or revise the original diagnosis and provide additional important information such as the tumour size, surgical margins, and spread of tumour cells to lymph nodes. Your pathologist will use this information to decide if the tumour is benign, borderline, or malignant and for your other doctors to decide if additional treatment is required.

What does a phyllodes tumour look like under the microscope?

A phyllodes tumour starts from the cells normally found in the breast. The tumour is made up of a mixture of fibroblasts surrounded by 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 different types of phyllodes tumours?

Pathologists divide phyllodes tumours into three types – benign, borderline, and malignant – based on the way the tumour looks when examined under the microscope. The type of tumour will help your doctor predict how the tumour will behave over time and select treatment options that are right for you.

Benign

A benign phyllodes tumour is a non-cancerous tumour. When examined under the microscope these tumours show no features of cancer (see sections on Stromal cellularity, Stromal overgrowth, Atypia, and Mitosis below). These tumours are usually treated with surgery alone. The tumour may grow back after surgery if the tumour is not completely removed (see Margins below).

Borderline

When examined under the microscope, a borderline phyllodes tumour has features that can be seen in both non-cancerous tumours and cancers. For this reason, a borderline tumour falls in between benign and malignant. Treatment for this type of tumour depends on the number of worrisome features seen. Most are treated with surgery alone, but some patients may be offered radiation therapy. The tumour may grow back after surgery if it is not completely removed (see Margins below).

Malignant

A malignant phyllodes tumour is considered a type of cancer. When examined under the microscope, this type of tumour has many features that are typically seen in cancer including:

• An increased number of tumour cells (see Stromal cellularity below).
• More stroma than epithelial cells (see Stromal overgrowth below).
• Abnormal looking tumour cells (see Atypia below).
• Dividing tumour cells (see Mitosis below).
• The edge of the tumour often blends with the normal surrounding breast tissue which makes it difficult to completely remove the tumour (see Tumour margin below).

A diagnosis of malignant phyllodes tumour usually requires your pathologist to see more than one of the features listed above. The diagnosis of a borderline tumour may be used if only one feature is seen or if the change is only seen in small areas of the tumour.

Malignant tumours can spread into the surrounding breast tissue and to other parts of the body. Patients with a malignant tumour may be offered radiation and chemotherapy after surgery.

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. Benign tumours have the least number of fibroblasts while malignant tumours have the most.

Stromal overgrowth 

Stromal overgrowth is used to describe a tumour that has areas made up entirely of stroma with no epithelial cells. Stromal overgrowth is not seen in a benign tumour. Small areas of stromal overgrowth can be seen in a borderline tumour while large areas of stromal overgrowth are common in malignant tumours. 

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 like those found in normal, healthy breast tissue. The cells in a borderline phyllodes tumour are atypical although they still share some features with normal, healthy cells. In contrast, the cells in a malignant phyllodes tumour are typically much larger, darker, and have irregular shapes compared to normal, healthy cells. Pathologists describe these cells as showing marked or significant cytologic atypia. 

Mitosis 

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.

Pathologists use established criteria for determining if the number of mitotic figures is consistent with a benign, borderline, or malignant phyllodes tumour. The criteria are:

• Benign – Less than 5 mitotic figures per 10 high powered fields.
• Borderline – 5 to 9 mitotic figures per 10 high powered fields.
• Malignant – 10 or more mitotic figures per 10 high powered fields. 

What is a surgical margin?

A surgical margin is any tissue that was cut by the surgeon to remove the tumour from your body. Whenever possible, surgeons will try to cut tissue outside of the tumour to reduce the risk that any tumour cells will be left behind after the tumour is removed. For phyllodes tumours, it is recommended that a surgical margin of at least 1 cm be present around the entire tumour to reduce the chance of recurrence.

Your pathologist will carefully examine all the margins in your tissue sample to see how close the tumour cells are to the edge of the cut tissue. Margins will only be described in your report after the entire tumour has been removed.

A negative margin means that no tumour cells were seen at the cut edge of the tissue. If all the margins are negative, most pathology reports will say how far the closest tumour cells were to a margin. The distance is usually described in millimetres. A positive margin means that tumour cells were seen at the cut edge of the tissue. A positive margin is associated with an increased risk that the tumour will grow back in the same site.

What is a tumour margin?

The tumour margin is the part of the tumour that touches the surrounding normal breast tissue. The margin between the tumour and the surrounding normal breast tissue is an important feature that helps your pathologist decide if the tumour is benign, borderline, or malignant.

• Benign – The tumour margin is clear, easy to see, and well separated from the surrounding normal breast tissue.
• Borderline – The tumour margin is mostly clear and easy to see although in some areas the tumour extends into the normal breast tissue in a way that makes it difficult to see the border.
• Malignant – The tumour spreads into the surrounding normal breast tissue in a way that makes it very difficult for your pathologist to see the tumour margin. Pathologists describe this as an infiltrative pattern of growth.

What does lymphovascular invasion mean?

Blood moves around the body through long thin tubes called blood vessels. Another type of fluid called lymph which contains waste and immune cells moves around the body through lymphatic channels. Tumour cells can use blood vessels and lymphatics to travel away from the tumour to other parts of the body. The movement of tumour cells from the tumour to another part of the body is called metastasis. Before tumour cells can metastasize, they need to enter a blood vessel or lymphatic. This is called lymphovascular invasion. 

Lymphovascular invasion will only be seen with malignant phyllodes tumours. Lymphovascular invasion increases the risk that tumour cells will be found in a lymph node or a distant part of the body such as the lungs.

What are lymph nodes?

Lymph nodes are small immune organs located throughout the body. Tumour cells can travel from the tumour to a lymph node through lymphatic channels located in and around the tumour. The movement of tumour cells from the tumour to a lymph node is called metastasis.

Tumour cells from malignant phyllodes tumours may spread to a lymph node. In contrast, tumour cells from a benign and borderline phyllodes tumour should not spread. For this reason, lymph nodes are not always removed at the time of surgery.

If lymph nodes are removed, each lymph node will be carefully examined for tumour cells. Lymph nodes that contain tumour cells are often called positive while those that do not contain any tumour cells are called negative. Most reports include the total number of lymph nodes examined and the number, if any, that contain tumour cells.

There are three types of lymph nodes that may be described in your report:

• Sentinel axillary lymph node – This is the first lymph node in the chain of lymph nodes that drains fluid from the breast. If tumour cells are going to be found in the axilla, they will usually be found in the sentinel node first.
• Non-sentinel axillary lymph node – This type of lymph node is located after the sentinel lymph node in the axilla. Tumour cells usually travel to these lymph nodes after passing through the sentinel lymph node.
• Internal mammary lymph node – This type of lymph node is found in the breast itself. Tumour cells may travel to these lymph nodes if the lymph node is found close to the tumour.

If tumour cells are found in a lymph node, the size of the area involved by the tumour will be measured and described in your report as follows:

• Isolated tumour cells – The area of tumour cells measures less than 0.2 millimetres and has less than 200 tumour cells.​
• Micrometastases – The area of tumour cells measures more than 0.2 millimetres but less than 2 millimetres.
• Macrometastases – The area of tumour cells measures more than 2 millimetres.