Metaplastic Carcinoma: Diagnosis, Treatment, and Outlook

Metaplastic breast carcinoma is a rare and aggressive form of invasive breast cancer, accounting for fewer than 5% of all breast cancer diagnoses, that stands apart because its tumor cells transform into tissue types not normally found in the breast, such as squamous cells, spindle cells, or cartilage-like and bone-like tissue.¹ This biological shape-shifting complicates every stage of clinical management, from the initial biopsy interpretation through treatment selection. Though it typically carries a triple-negative receptor profile, metaplastic carcinoma tends to respond less well to standard chemotherapy than ordinary triple-negative disease, and its prognosis is correspondingly worse.

How It Differs from Common Breast Cancers

Most breast cancers arise from the ductal or lobular cells that line the milk-producing structures, and those cells remain recognizably “epithelial” under the microscope. In metaplastic carcinoma, some or all of the cancer cells undergo a transformation into cell types the breast does not normally contain. You might see squamous epithelium (the kind that lines your skin and throat), spindle-shaped cells resembling connective tissue, or even cartilage and bone matrix scattered through the tumor. A pathologist reviewing a biopsy may find several of these elements mixed together in a single mass.

In a study of 97 cases, roughly half were a mixture of conventional carcinoma and metaplastic components, with the metaplastic portion ranging widely from as little as 5% to as much as 95% of the tumor. The most common pure subtype in that series was matrix-producing (tumors making cartilage or bone-like material), followed by squamous and spindle cell varieties.² Matrix-producing tumors often appeared as well-defined nodules with central areas of dead tissue, a pattern that can be mistaken for a benign lesion on imaging.

Underlying this transformation is a process called epithelial-to-mesenchymal transition, in which cancer cells lose their epithelial identity and acquire properties more typical of mesenchymal (connective tissue) cells. This shift is partly driven by signaling through the TGF-β pathway, which in normal tissue suppresses cell growth but in advancing cancer paradoxically promotes invasion and treatment resistance.³ The result is a tumor that behaves more like a sarcoma in some ways while retaining the genetic fingerprint of a carcinoma.

Clinical Presentation and Imaging

Metaplastic breast carcinoma tends to show up as a fast-growing, palpable mass. Compared with the more common invasive ductal carcinoma, it presents at a larger size, is more likely to be triple-negative (lacking estrogen receptor, progesterone receptor, and HER2), and often has high Ki-67 expression, a marker of rapid cell division. Somewhat paradoxically, lymph node involvement at diagnosis is lower than in ordinary breast cancer.⁴ A meta-analysis found that metaplastic tumors tend to arise in patients aged 50 or older, often exceed 5 cm in diameter, and are frequently lymph node-negative at diagnosis.⁵

On mammography, these tumors usually appear as dense, round or oval masses, often without the spiculated (star-shaped) borders that radiologists associate with malignancy. Calcifications are uncommon, which can lead to a lower suspicion of cancer at screening. Ultrasound tends to reveal a complex mass with both solid and fluid-filled areas, a reflection of the internal necrosis and cystic degeneration that metaplastic tumors frequently undergo. Posterior acoustic enhancement, a feature more commonly associated with benign cysts, is also frequently seen.⁶ ⁷ A retrospective imaging review found that about three-quarters of cases appeared high-density on mammography, while on ultrasound, the majority were oval-shaped and hypoechoic.⁸

In rare instances, the tumor may not even originate from a detectable mass in the breast itself. One case report described a 40-year-old woman whose only finding was an axillary mass that initially appeared muscular in origin on ultrasound. Only needle biopsy revealed spindle cell atypia, leading eventually to a metaplastic breast cancer diagnosis.⁹

The Diagnostic Challenge

Getting the diagnosis right matters enormously because metaplastic carcinoma can masquerade under the microscope as a range of other conditions, from benign scars and fibromatosis to malignant phyllodes tumors and primary sarcomas.¹⁰ Especially in small core needle biopsies, the spindle-cell component may look almost identical to a phyllodes tumor or connective tissue proliferation. Cytokeratin staining, the usual first step, can be patchy and unreliable in metaplastic carcinoma, so a single negative keratin stain does not rule it out.¹¹

Pathologists rely on a panel of immunohistochemical stains to build the case. Broad-spectrum cytokeratins are positive in roughly 80% of metaplastic carcinomas, while basal cytokeratins are positive in about 70%. Luminal cytokeratins show up in only about 30 to 60% of cases. Estrogen receptor, progesterone receptor, and HER2 are usually all negative.¹² One stain that has proven especially useful is p63: it was strongly positive in about 87% of metaplastic carcinomas in one study (all 12 cases with spindle or squamous features were positive), while fewer than 1% of non-metaplastic invasive breast cancers stained for it. Phyllodes tumors and sarcomas were consistently negative, giving p63 both high sensitivity and high specificity for metaplastic carcinoma.¹³

There is an important caveat: a subset of malignant phyllodes tumors can also show focal p63 and cytokeratin staining, so a positive p63 result on a core biopsy alone does not absolutely settle the diagnosis.¹⁴ CD34, a marker frequently positive in phyllodes tumors, is consistently negative in metaplastic carcinoma, making it a helpful differentiator in ambiguous cases.¹²

Receptor Status and Why It Matters

About two-thirds of metaplastic breast cancers are triple-negative, meaning they lack the receptors that would make them candidates for hormonal therapy or HER2-targeted drugs.¹⁵ A smaller fraction, roughly a quarter, express one or both hormone receptors but are HER2-negative, and about 5% are HER2-positive. These minority subgroups open the door to standard targeted agents, but the clinical behavior of metaplastic tumors with a single positive hormone receptor appears to track more closely with triple-negative disease than with typical hormone-positive cancers. One SEER-based analysis found that single hormone-receptor-positive metaplastic tumors had outcomes similar to the triple-negative subtype.¹⁶ The practical takeaway is that hormone receptor positivity in metaplastic carcinoma should not be assumed to carry the same reassurance it would in a more common breast cancer type.

The Genomic Landscape

Genomic profiling has revealed a distinctive mutation pattern that helps explain both the aggressive behavior and potential treatment vulnerabilities of metaplastic carcinoma. The two most common alterations involve the tumor suppressor gene TP53 and the PI3K signaling pathway. TP53 mutations appear in roughly 65 to 70% of cases, while PI3K pathway alterations (primarily PIK3CA mutations and PTEN losses) are found in a similar proportion.¹⁷ Compared with ordinary triple-negative invasive ductal carcinoma, metaplastic tumors are enriched for PIK3CA mutations, as well as mutations in ARID1A, FAT1, and PTEN.¹⁸

Beyond point mutations, metaplastic carcinomas show activation of several interconnected signaling networks, including epidermal growth factor receptor (EGFR) amplification, Wnt/β-catenin signaling, and cell cycle dysregulation.¹⁹ The high frequency of PI3K pathway changes is especially relevant because drugs targeting this pathway already exist and are being tested in this disease.

Surgery and Radiation

Historically, patients with metaplastic carcinoma have been more likely to undergo mastectomy than breast-conserving surgery, in part because these tumors tend to be large at diagnosis. In a national database analysis of about 2,500 metaplastic breast cancer patients, roughly 59% received mastectomy compared with 45% of non-metaplastic breast cancer patients.²⁰ However, a recent systematic review and meta-analysis of studies comparing breast conservation with mastectomy in metaplastic carcinoma found no significant difference in overall survival between the two approaches.²¹ When tumor size and margins allow, breast conservation appears to be a viable option.

Radiation therapy following lumpectomy appears to improve outcomes. One population-based analysis found that patients who had lumpectomy with radiation had a ten-year overall survival of about 66%, compared with roughly 59% for those who had lumpectomy without radiation.²² Smaller institutional series have reported similar trends: local recurrence after lumpectomy dropped from about 25% without radiation to under 8% with it.²³ The benefit of post-mastectomy radiation is less clear; some data suggest an overall survival advantage, but the effect on disease-specific survival has been harder to demonstrate.

One counterintuitive finding from the same national database analysis is that axillary lymph node dissection was associated with decreased survival in metaplastic carcinoma patients, even though it was performed in about a third of cases. Metaplastic tumors spread to lymph nodes less often than other breast cancers, so the morbidity of a full dissection may not be justified unless nodes are clearly involved. Sentinel lymph node biopsy, a less invasive sampling technique, is generally favored as the initial assessment.²⁰

Chemotherapy and Its Limitations

Standard anthracycline-and-taxane-based chemotherapy regimens are the most commonly used systemic treatment, but metaplastic carcinoma has earned a reputation for being relatively chemoresistant.²⁴ When compared head-to-head with ordinary triple-negative breast cancer treated with the same regimens, metaplastic carcinoma shows significantly worse progression-free and overall survival. One study found three-year progression-free survival of roughly 51% for metaplastic cases versus 82% for triple-negative non-metaplastic cancers, with a similar gap in overall survival.²⁵

There is evidence, however, that platinum-based chemotherapy may be more effective than traditional regimens. In a single-institution retrospective series, all nine triple-negative metaplastic carcinoma patients who received a combination of platinum and taxane as adjuvant therapy were alive and disease-free after more than five years of follow-up, with a median follow-up exceeding eight years.²⁶ These numbers come from a small, uncontrolled study, so they should be interpreted cautiously, but they align with a broader trend suggesting that platinum agents may be particularly well suited to this tumor type. Many oncologists now incorporate platinum drugs into the treatment plan for metaplastic carcinoma, particularly in the neoadjuvant or adjuvant setting.

Targeted and Emerging Therapies

The high prevalence of PI3K pathway mutations in metaplastic carcinoma has prompted clinical investigation of drugs targeting this signaling cascade. In a trial combining an mTOR inhibitor with liposomal doxorubicin and bevacizumab, 52 patients with metaplastic breast cancer achieved an objective response rate of 21% and a clinical benefit rate (including stable disease lasting six months or more) of 40%. Patients whose tumors harbored PI3K pathway mutations had a significantly higher response rate of 31%, compared with 0% in patients without such mutations.²⁷ This finding underscores the value of genomic profiling: knowing whether a PI3K mutation is present can guide the choice between a targeted regimen and standard chemotherapy.

Immunotherapy has also generated interest. About half of metaplastic carcinomas show PD-L1 expression on immune cells infiltrating the tumor, though this varies substantially by subtype. Squamous components tend to have the highest rates of immune cell infiltration and PD-L1 positivity, while matrix-producing components tend to have the lowest.²⁸ Case reports have documented dramatic responses to checkpoint inhibitors like pembrolizumab, particularly when combined with chemotherapy. One patient with metastatic metaplastic carcinoma whose lung metastases were PD-L1-positive had significant tumor shrinkage after pembrolizumab plus gemcitabine/carboplatin, even though treatment was stopped early due to side effects.²⁹ A notable finding from the immune profiling work is that high levels of tumor-infiltrating lymphocytes correlated with better survival, independent of PD-L1 status, suggesting that the immune system plays a meaningful role in controlling these tumors even without immunotherapy drugs.²⁸

Antibody-drug conjugates represent another emerging option. Sacituzumab govitecan, which targets the Trop-2 protein on the surface of cancer cells and delivers a potent chemotherapy payload directly, is already approved for triple-negative breast cancer after prior treatment. Case reports have described sustained responses lasting close to a year in patients with metastatic metaplastic carcinoma who had progressed on multiple other treatments.³⁰ However, a larger retrospective analysis comparing sacituzumab govitecan in metaplastic versus non-metaplastic triple-negative breast cancer found a shorter median progression-free survival in the metaplastic group (about two months versus nearly six months), although overall survival was similar between the two groups.³¹ That same analysis found that roughly 60% of metaplastic tumors were HER2-low, potentially making them candidates for trastuzumab deruxtecan, another antibody-drug conjugate.

Prognosis and Patterns of Spread

Metaplastic breast carcinoma carries a worse prognosis than both ordinary invasive ductal carcinoma and standard triple-negative breast cancer.¹⁹ An individual patient-level meta-analysis spanning nearly five decades of published cases reported a median overall survival of about 75 months and a median progression-free survival of about 36 months. Tumor size, lymph node involvement, distant metastasis, and histologic subtype were all significant predictors of outcome.³²

When metaplastic carcinoma does spread beyond the breast, it shows a pattern that diverges somewhat from typical breast cancer. The lungs are the most common site of distant metastasis, reported in roughly 22 to 70% of patients who develop metastatic disease, followed by bone and then liver.³³ ³⁴ Brain metastases, while less frequent, were observed in about 7% of cases in one series. The relatively low rate of lymph node involvement at diagnosis but high rate of distant hematogenous spread fits with the mesenchymal transformation these cells undergo, as connective tissue-type cells tend to travel through the bloodstream rather than the lymphatic system.

Circulating Tumor DNA and Future Monitoring

One area with practical implications for the near future is the use of circulating tumor DNA (ctDNA) as a blood-based monitoring tool. In breast cancer generally, detectable ctDNA after surgery or during treatment has been linked to worse outcomes. The most frequently identified actionable mutations in ctDNA include PTEN, PIK3CA, and HER2, all of which are relevant to metaplastic carcinoma’s genomic profile.³⁵ For a tumor type that is already hard to monitor because it may not light up reliably on standard imaging or because lymph node status is a poor proxy for systemic risk, a blood test that can detect residual disease and flag targetable mutations at the same time is an attractive prospect. This is not yet standard practice for metaplastic carcinoma specifically, but clinical trials are increasingly incorporating ctDNA endpoints, and it is plausible that liquid biopsies will become part of routine follow-up within the next several years.

Why Subtype Identification Matters for Treatment

Not all metaplastic carcinomas behave the same way, and the specific histologic subtype can influence both immune response and treatment sensitivity. Squamous-predominant tumors, for instance, tend to attract more immune cells and express higher levels of PD-L1, which may make them better candidates for immunotherapy. Matrix-producing tumors, by contrast, appear to inhabit a more immunologically “cold” microenvironment.²⁸ Spindle cell tumors occupy a middle ground and may show PD-L1 expression on the tumor cells themselves, a feature less common in other subtypes.

PI3K pathway mutation frequency also varies by subtype.¹⁷ Because targeted therapies work only when the relevant mutation is present, comprehensive genomic profiling at diagnosis, rather than relying solely on standard receptor testing, can meaningfully expand the list of treatment options. The retrospective analysis from ASCO 2025 found that among 13 metaplastic cases tested, 5 had actionable PIK3CA mutations, 2 had PTEN alterations, and 8 were HER2-low, each pointing toward a different approved or investigational therapy.³¹ In a disease where standard chemotherapy often underperforms, identifying these vulnerabilities at the outset is one of the most productive steps a treatment team can take.

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