Your pathology report for benign phyllodes tumor of the breast

by Jason Wasserman MD PhD FRCPC
May 23, 2025

A benign phyllodes tumor is a rare, non-cancerous breast tumor that arises from the stroma (supportive connective tissue) within the breast. Unlike borderline or malignant types, benign phyllodes tumors grow slowly. They do not spread beyond their original location or to other parts of the body.

What causes a benign phyllodes tumors?

The exact cause of benign phyllodes tumors is unknown. Genetic changes, such as mutations in the MED12 gene, have been associated with their development. Benign phyllodes tumors may sometimes develop from pre-existing breast lumps like fibroadenomas.

What are the symptoms of a benign phyllodes tumors?

Benign phyllodes tumors usually present as firm, painless breast lumps that grow slowly over time. These lumps can become quite large and may distort the shape of the breast or stretch the skin, but typically do not cause significant discomfort or other symptoms.

How is this diagnosis made?

Diagnosis typically involves imaging studies such as mammography, ultrasound, or MRI, followed by a biopsy. A tissue sample is taken during the biopsy and examined by a pathologist under a microscope. Pathologists confirm benign phyllodes tumors by identifying specific microscopic features, including mild cellular growth and minimal cell abnormalities.

Microscopic features

Under the microscope, benign phyllodes tumors exhibit mild stromal cellularity, minimal cell abnormalities (atypia), and low mitotic activity (rate of cell division). They often show well-defined leaf-like structures lined with benign epithelial cells. Pathologists distinguish benign phyllodes tumors from borderline or malignant types primarily based on these features, noting the absence of aggressive characteristics like marked stromal overgrowth or infiltration.

phyllodes tumor

Tumor extension

Benign phyllodes tumors typically remain confined within the breast tissue, growing as clearly defined, rounded, or oval masses. They do not invade deeper breast tissues or spread to nearby structures or distant parts of the body.

Margins

Margins refer to the edges of the surgically removed breast tissue. Assessing margins is important for ensuring complete tumor removal:

  • Negative margins: No tumor cells are present at the edges, indicating complete removal.
  • Positive margins: Tumor cells are found at the edges, suggesting potential remaining tumor tissue, although benign tumors rarely cause significant concern even if margins are positive.

Clear (negative) margins are generally preferred to reduce the chance of recurrence.

Margin

Prognosis for benign phyllodes tumor

Benign phyllodes tumors have an excellent prognosis, with very low risks of recurrence or progression to malignant forms. Surgical removal with clear margins typically provides a complete cure. Regular monitoring is recommended to ensure no recurrence.

Questions to ask your doctor

  • How certain is the diagnosis of benign phyllodes tumor?
  • What are the surgical options, and how extensive will the surgery need to be?
  • Is any additional treatment required after surgery?
  • What are the chances of the tumor recurring?
  • How often should I have follow-up exams or imaging studies?
  • Are there any lifestyle changes or precautions I should consider after surgery?
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