Metaplastic breast cancer is a rare, aggressive form of breast cancer that accounts for less than 1% of all breast cancer diagnoses. What sets it apart is that the cancer cells transform into types not normally found in the breast, such as cells resembling skin, cartilage, or bone tissue. This unusual cellular behavior is what gives the disease its name: “metaplasia” means a change from one cell type to another.
How It Differs From Common Breast Cancers
Most breast cancers begin in the milk ducts or lobules and remain recognizable as breast tissue under a microscope. Metaplastic breast cancer starts the same way but then undergoes a transformation. The cancer cells differentiate into squamous cells (the flat cells that make up skin), spindle-shaped cells, or mesenchymal elements like cartilage and bone. This mix of cell types within a single tumor is one reason it can be difficult to diagnose and treat.
The vast majority of metaplastic breast cancers are triple-negative, meaning they lack the three receptors (estrogen, progesterone, and HER2) that many breast cancer treatments target. This limits the standard hormonal and targeted therapy options available for more common breast cancers, making treatment planning more complex from the start.
Subtypes of Metaplastic Breast Cancer
The World Health Organization recognizes several distinct subtypes, grouped by how aggressive they tend to be. The four aggressive subtypes are:
- Spindle cell carcinoma: cancer cells elongate into a spindle shape, sometimes mimicking a soft tissue tumor rather than a breast cancer
- Squamous cell carcinoma: cells resemble the flat cells found in skin
- Metaplastic carcinoma with mesenchymal differentiation: the tumor produces cartilage, bone, or muscle-like tissue
- Mixed metaplastic carcinoma: a combination of two or more of the above patterns
Two additional subtypes, low-grade adenosquamous carcinoma and fibromatosis-like metaplastic carcinoma, tend to behave less aggressively and carry a more favorable outlook.
Diagnosis and What Makes It Tricky
Because the tumor cells can look like entirely different tissue types, metaplastic breast cancer is sometimes misidentified on an initial biopsy. A pathologist may mistake a spindle cell variant for a soft tissue sarcoma, or a squamous variant for a cancer that originated somewhere other than the breast.
To confirm the diagnosis, pathologists use specialized staining techniques. One particularly useful marker is a protein called p63, which lights up in metaplastic carcinoma cells but not in pure sarcomas or standard breast carcinomas. Testing for p63 has a specificity of 96%, meaning that when it’s positive, it almost certainly points to metaplastic carcinoma rather than a lookalike. Its sensitivity is lower, around 65%, so a negative result doesn’t rule out the diagnosis entirely. Additional stains for epithelial and mesenchymal markers help complete the picture.
These tumors also tend to be larger at the time of diagnosis than more common breast cancers, partly because they can grow quickly and partly because their unusual appearance on imaging may delay recognition.
Treatment Approach
Surgery is the primary treatment. Both mastectomy and breast-conserving surgery (lumpectomy followed by radiation) are options, depending on tumor size and location. The standard margin goal for breast-conserving surgery is “no ink on tumor,” meaning no cancer cells are touching the edge of the removed tissue. If cancer is found at the margin, a second surgery is typically recommended, though the decision factors in the extent of involvement and individual circumstances.
Chemotherapy is commonly used, but metaplastic breast cancer is widely considered less responsive to standard chemotherapy regimens than other triple-negative breast cancers. This relative resistance is one of the defining clinical challenges of the disease. Oncologists may try different drug combinations, but response rates are generally lower than what would be expected for a triple-negative cancer of comparable size and stage.
Because most metaplastic tumors are triple-negative, hormonal therapies like tamoxifen and aromatase inhibitors are ineffective. HER2-targeted drugs similarly have no role in the majority of cases.
Immunotherapy as a Treatment Option
One area of growing interest is immunotherapy. Many metaplastic breast cancers express a protein called PD-L1 on their surface. PD-L1 acts as a “don’t attack me” signal that cancer cells use to hide from the immune system. Drugs that block this signal, called immune checkpoint inhibitors, have shown encouraging results in triple-negative breast cancer overall, particularly when combined with chemotherapy.
For metaplastic breast cancer specifically, the evidence is still developing. Because these tumors often have higher PD-L1 expression than other breast cancers, there is reason to believe checkpoint inhibitors could be especially relevant. Some patients with metaplastic disease have been treated with these drugs in clinical trials and broader triple-negative protocols, but large studies focused solely on metaplastic breast cancer are limited by the disease’s rarity.
Prognosis and Recurrence
A retrospective study spanning 14 years found a five-year disease-free survival rate of about 74% and a five-year overall survival rate of roughly 83% for metaplastic breast cancer patients. These numbers are generally lower than for the most common type of breast cancer, invasive ductal carcinoma, reflecting the disease’s aggressive biology and limited treatment sensitivity.
Prognosis varies significantly by subtype. The low-grade adenosquamous and fibromatosis-like variants carry a much better outlook than the spindle cell or mesenchymal subtypes. Tumor size at diagnosis and whether the cancer has spread to lymph nodes also play major roles.
When recurrence happens, it most commonly occurs within the first five years after surgery. Local recurrence means the cancer returns near the original site, while distant recurrence can involve the lungs, bones, liver, or brain. Metaplastic breast cancer has a tendency toward distant spread, sometimes to sites less typical for breast cancer, such as the lungs, which may relate to its mesenchymal characteristics.
Why Specialized Care Matters
Because metaplastic breast cancer is so uncommon, many community oncologists may see only a handful of cases in their careers. Treatment decisions benefit from input at a cancer center with experience in rare breast cancers, where pathologists are more familiar with the diagnostic challenges and oncologists have a broader toolkit for managing triple-negative, chemotherapy-resistant disease. Getting the pathology reviewed by a specialist is one of the most impactful steps a patient can take early in the process, since an accurate subtype classification directly shapes the treatment plan and expected outcomes.