Breast Mucoepidermoid Carcinoma with a Rare CRTC3–MAML2 Fusion

Mucoepidermoid carcinoma is the most prevalent malignancy in the salivary gland and is sporadic in the breast. Here, we report a case of breast mucoepidermoid carcinoma with a rare CREB-regulated transcription coactivator 3–mastermind-like transcriptional coactivator 2 (CRTC3–MAML2) fusion. A 23-year-old female was admitted to our hospital with a left breast palpable mass. Histologic findings of the core-needle biopsy indicated breast cancer. The section revealed a squamoid tumor-cell proliferation with enlarged nuclei and eosinophilic cytoplasm among smaller intermediate cells and abundant cystic spaces containing secretory materials. The features were compatible with mucoepidermoid carcinoma in low-grade, confirmed by detecting the CRTC3–MAML2 fusion using reverse transcription polymerase chain reaction and direct sequencing. We only administered tamoxifen postoperatively without other adjuvant therapy because her tumor partially expressed hormonal receptors. No signs indicate a recurrence or metastasis in our over 3 year follow-up. The genetic analysis helps in definitively diagnosing breast mucoepidermoid carcinoma, and the treatment strategy should be considered based on the histologic findings.