By Jason Wasserman MD PhD FRCPC
August 16, 2023
A borderline phyllodes tumour is a type of breast tumour that shows uncertain behavior over time. While most borderline phyllodes will behave like a non-cancerous tumour over time, a small number will spread to other parts of the body in a manner more in keeping with a cancerous tumour.
Borderline phyllodes tumours usually present as a round, painless lump that you or your doctor can feel during a breast examination. 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.
A phyllodes tumour is diagnosed as borderline when it has microscopic features that overlap between a benign phyllodes tumour and a malignant phyllodes tumour. Over time, some borderline phyllodes tumours can change to develop more malignant (cancerous) features and behaviour more in keeping with a malignant phyllodes tumour.
Although rare, borderline phyllodes tumours can metastasize (spread) to other parts of the body. The most common sites for metastasis are the lungs and bones. The tumour cells in a borderline phyllodes tumour very rarely metastasize to lymph nodes.
At the present time, doctors do not know what causes a borderline phyllodes tumour to develop.
The diagnosis of a borderline 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. When a biopsy is performed, a borderline phyllodes tumour may be initially diagnosed as a fibroepithelial lesion because it cannot be distinguished from similar tumours such as the more common fibroadenoma.
Like all phyllodes tumours, a borderline 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’.
A phyllodes tumour is diagnosed as borderline when it shows some but not all the microscopic features associated with a malignant phyllodes tumour. Specifically, pathologists look for four microscopic features when making the diagnosis: stromal cellularity, stromal overgrowth, stromal atypia, mitotic activity, and the pattern of growth. Each of these features are discussed in more detail below.
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 borderline phyllodes tumour, the stromal cellularity is usually greater than in a benign phyllodes tumour but is less than in a malignant phyllodes tumour.
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. Borderline phyllodes tumours may show very small areas of stromal overgrowth.
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 stromal cells to decide if they are atypical. The stromal cells in a borderline phyllodes tumour may show mild to moderate atypia.
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. It is common for a borderline phyllodes tumour to have mitotic figures with a rate between 2 and 5 mitotic figures per 10 high-powered fields. In contrast, mitotic figures are usually found throughout a malignant phyllodes tumour with a rate exceeding 5 per 10 high-powered fields.
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 borderline 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. However, some areas of the tumour may show a permeative pattern of growth which means the tumour is extending into the surrounding normal breast tissue.
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 borderline phyllodes tumour, 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.