Borderline phyllodes tumour of the breast

By Jason Wasserman MD PhD FRCPC
May 23, 2025

A borderline phyllodes tumor is an uncommon breast tumor that arises from the stroma (supportive connective tissue) within the breast. It has characteristics that fall between benign (non-cancerous) and malignant (cancerous) phyllodes tumors. Borderline phyllodes tumors grow faster and have a greater likelihood of recurrence than benign tumors, but they typically do not spread to distant parts of the body.

What causes borderline phyllodes tumors?

The exact cause of borderline phyllodes tumors is unknown. Genetic mutations, particularly in genes such as MED12, have been linked to these tumors. Some borderline phyllodes tumors may develop from pre-existing benign breast lumps, such as fibroadenomas, through genetic changes.

What are the symptoms of borderline phyllodes tumors?

Borderline phyllodes tumors typically present as firm, painless, and often rapidly enlarging lumps in the breast. While usually smaller than malignant phyllodes tumors, they can still become large enough to stretch or distort the breast skin, sometimes causing discomfort.

How is borderline phyllodes tumor diagnosed?

Diagnosis usually begins with imaging tests such as mammography, ultrasound, or MRI, followed by a biopsy. During the biopsy, a sample of tissue is examined by a pathologist under a microscope. Borderline phyllodes tumors are identified by intermediate stromal abnormalities, moderate cellular growth, and clear but less aggressive patterns of invasion compared to malignant tumors.

Microscopic features

Under the microscope, borderline phyllodes tumors display intermediate features between benign and malignant tumors. Pathologists look for moderate stromal cellularity, some cellular abnormalities (atypia), and moderate mitotic activity (rate of cell division). Unlike malignant tumors, borderline phyllodes tumors typically do not show extensive stromal overgrowth or marked infiltration into surrounding breast tissue, but their microscopic appearance is more aggressive than benign phyllodes tumors.

Tumor extension

Borderline phyllodes tumors originate within breast tissue and may grow to a large size, occasionally distorting or stretching the breast. While these tumors do not commonly invade deeper breast tissue layers extensively or spread to distant organs, local growth can sometimes affect nearby breast structures and skin.

Margins

Margins refer to the edges of the surgically removed breast tissue. Evaluating these margins helps ensure the tumor has been completely removed:

  • Negative margins: No tumor cells are found at the edges, indicating the entire tumor has been removed.
  • Positive margins: Tumor cells are present at the edges, suggesting that some tumor tissue may remain and potentially increasing the risk of recurrence.

Achieving clear (negative) margins is essential in reducing the likelihood of recurrence for borderline phyllodes tumors.

Lymphovascular invasion

Lymphovascular invasion means the presence of tumor cells within lymphatic or blood vessels, which could indicate a higher potential for spreading. However, borderline phyllodes tumors rarely show lymphovascular invasion or spread to lymph nodes.

Prognosis for borderline phyllodes tumor

Borderline phyllodes tumors have a prognosis between benign and malignant types. While they generally have a favorable outlook, there is a moderate risk of recurrence, particularly if the tumor has positive margins after surgery. Complete surgical removal with clear margins is typically sufficient treatment. Due to the potential for recurrence, ongoing monitoring and follow-up are important.

Questions to ask your doctor

  • What is the risk that my borderline phyllodes tumor will come back?
  • What treatment options do you recommend?
  • How frequently should I have follow-up exams and imaging?
  • What signs should prompt me to seek immediate evaluation after treatment?
  • Are there any clinical trials or newer treatments suitable for my tumor type?
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