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.