Malignant phyllodes tumor

by Jason Wasserman MD PhD FRCPC
May 23, 2025

A malignant phyllodes tumor is a rare type of breast cancer that arises from the stroma (supportive connective tissue) of the breast. Unlike more common types of breast cancer, phyllodes tumors have both epithelial (ductal or glandular) and stromal (connective tissue) components. Malignant phyllodes tumors are characterized by rapid growth, significant cellular abnormalities, and a tendency to recur or spread beyond the breast.

What causes malignant phyllodes tumors?

The exact cause of malignant phyllodes tumors is unknown. However, certain genetic mutations and alterations in genes such as MED12, TP53, and EGFR have been associated with these tumors. Malignant phyllodes tumors can sometimes develop from pre-existing benign fibroepithelial tumors like fibroadenomas through genetic changes.

What are the symptoms of malignant phyllodes tumors?

Malignant phyllodes tumors typically present as firm, painless, and rapidly enlarging breast lumps. Larger tumors may stretch the skin, causing distortion or ulceration. Occasionally, tumors may break through the skin surface, forming visible, fleshy growths. Symptoms like bloody nipple discharge or reactive swelling of the lymph nodes may occur, but these are uncommon.

How is a malignant phyllodes tumor diagnosed?

Diagnosis typically involves imaging studies such as mammography, ultrasound, or MRI, followed by a biopsy. During a biopsy, a tissue sample is taken and examined by a pathologist under a microscope. Pathologists identify malignant phyllodes tumors by the presence of highly abnormal, rapidly dividing stromal cells, significant stromal overgrowth, and infiltration into surrounding breast tissue.

Microscopic features

Under the microscope, malignant phyllodes tumors show distinct characteristics that help pathologists confirm the diagnosis. Key criteria include marked stromal cellular abnormalities (pleomorphism), high stromal cell density, and high mitotic activity (frequent cell division). A hallmark feature is stromal overgrowth, where large areas contain only stromal cells without any epithelial cells. Additionally, malignant phyllodes tumors typically have an infiltrative growth pattern, meaning they invade surrounding breast tissue rather than forming well-defined borders. Pathologists distinguish malignant phyllodes tumors from benign or borderline types primarily based on the degree of these features: benign tumors have lower stromal cellularity and minimal atypia, while borderline tumors display intermediate characteristics.

Tumor extension

Malignant phyllodes tumors originate within breast tissue and can invade adjacent tissues, including surrounding breast structures and skin. They commonly infiltrate through the breast stroma, potentially affecting deeper breast tissue layers and, in advanced cases, spreading to nearby muscles or chest wall tissues. Although uncommon, these tumors may also spread to distant organs such as the lungs and bones.

Margins

Margins refer to the edges of the surgically removed breast tissue. The status of these margins is crucial for determining if the tumor has been completely removed:

• Negative margins: No cancer cells are present at the tissue edges, indicating the tumor has been fully removed.
• Positive margins: Cancer cells are present at the edges, suggesting some tumor tissue may remain and increasing the risk of recurrence.

Clear (negative) margins reduce the likelihood of tumor recurrence, whereas positive margins often require additional treatment such as further surgery or radiation therapy.

Lymphovascular invasion

Lymphovascular invasion means the presence of tumor cells within lymphatic or blood vessels, indicating a higher likelihood of the tumor spreading beyond the breast. However, lymph node metastasis is rare in malignant phyllodes tumors, making routine lymph node removal uncommon.

AJCC stage

The American Joint Committee on Cancer (AJCC) staging system categorizes malignant phyllodes tumors based on tumor size (T category), lymph node involvement (N category), and distant spread (M category):

T category (Tumor size)

• pT1: The tumor measures 5 cm or less in greatest dimension.
• pT2: The tumor is larger than 5 cm but no more than 10 cm.
• pT3: The tumor is larger than 10 cm but no more than 15 cm.
• pT4: The tumor is larger than 15 cm.

N category (Lymph node involvement)

• pN0: No regional lymph node involvement.
• pN1: Cancer cells have spread to regional lymph nodes.

M category (Distant metastasis)

• pM0: No distant spread.
• pM1: Cancer cells have spread to distant organs, such as the lungs or bones.

Prognosis for malignant phyllodes tumor

Malignant phyllodes tumors have a more aggressive clinical course compared to benign or borderline phyllodes tumors. The risk of recurrence is higher, especially if the tumor was incompletely removed (positive margins). Distant metastasis is uncommon but possible, usually occurring within 5 to 8 years following diagnosis. Treatment typically includes wide surgical removal, and in some cases, radiation therapy or chemotherapy may be considered.

Questions to ask your doctor

• What stage is my malignant phyllodes tumor?
• What are the surgical options available, and how extensive should the surgery be?
• Will I need additional treatments such as radiation therapy or chemotherapy?
• What is my risk of recurrence?
• What symptoms should I watch for following treatment?
• How frequently should I have follow-up exams and imaging studies?
• Are there clinical trials or new treatments that may be suitable for my condition?