Metaplastic carcinoma of the breast

Metaplastic carcinoma of the breast (also known as metaplastic breast carcinoma or MBC) is a rare and aggressive subtype of breast cancer in which the typical glandular (adenocarcinoma) cells of the breast undergo metaplasia — a transformation into non-glandular cell types such as squamous cells, spindle cells, or mesenchymal elements including bone, cartilage, or muscle-like tissue. This heterogeneous group of tumors accounts for less than 1% of all invasive breast cancers. Metaplastic carcinoma primarily affects the breast but can metastasize to the lungs, bones, liver, and brain. It tends to present as a rapidly growing breast mass that may be large at diagnosis. Clinically, metaplastic carcinoma of the breast is notable for its typically triple-negative receptor status, meaning the tumor cells usually lack expression of estrogen receptors, progesterone receptors, and HER2. This limits the effectiveness of hormonal therapies and HER2-targeted agents. The disease tends to have a poorer prognosis compared to other forms of invasive breast carcinoma of similar stage, with higher rates of local recurrence and distant metastasis. Subtypes include squamous cell carcinoma, spindle cell carcinoma, carcinosarcoma, and mixed metaplastic carcinoma, among others. Treatment typically involves surgical excision (mastectomy or wide local excision), often combined with radiation therapy. Chemotherapy is commonly used, though response rates tend to be lower than in conventional triple-negative breast cancers. Research into targeted therapies, including PI3K/mTOR pathway inhibitors and immune checkpoint inhibitors, is ongoing, as these tumors frequently harbor PI3KCA mutations and may show PD-L1 expression. Due to its rarity, management is often guided by institutional experience and emerging clinical trial data rather than large randomized studies.

Data sourced from FDA regulatory filings and ClinicalTrials.gov. Updated periodically.

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