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.
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.
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.
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.
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.
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 refer to the edges of the surgically removed breast tissue. Evaluating these margins helps ensure the tumor has been completely removed:
Achieving clear (negative) margins is essential in reducing the likelihood of recurrence for borderline phyllodes tumors.
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.
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.