Primary pulmonary epithelial-myoepithelial carcinoma: Report of a rare and under-diagnosed low-grade malignancy.

Primary epithelial-myoepithelial carcinoma (EMC) is a rare low-grade malignant neoplasm of the lung that originates from the submucosal bronchial glands. It behaves in an indolent fashion, although rare cases with high-grade transformation have been reported. Because of the rarity, optimal therapy for this entity has not been clearly defined. Herein, we report a case of primary pulmonary EMC in a 38-year-old Indian man who had a short history of dyspnea and a computed tomographic (CT) scan revealed a 3 cm diameter homogeneous mass in the lower lobe of the right lung. A CT-guided biopsy revealed a mildly atypical and mitotically quiescent tumor with solid and focal acinar arrangement; foci with biphasic arrangement by inner epithelial and outer myoepithelial cells were identified. The neoplasm revealed cytokeratin (CK) 7 positivity in the epithelial cells, while the myoepithelium expressed smooth muscle actin and p63. The tumor had a low (8%) Ki-67 proliferation index. The neuroendocrine markers, thyroid transcription factor 1, CK5/6, p40, and napsin A were negative. Positron emission tomography-CT was negative for any other mass lesion. The mass was excised with negative margins and the patient was on close follow without any evidence of disease for the past 17 months. A custom made, targeted DNA- and RNA-based 5 gene lung cancer next-generation sequencing panel (Epidermal growth factor receptor (EGFR), anaplastic lymphoma kinase (ALK), Ros protocol-oncogene 1 tyrosine kinase (ROS1), B-rapidly accelerated fibrosarcoma family oncogene (BRAF), and mesenchymal epithelial transition molecule (MET)), compatible with the Ion S5 system was performed; however, no mutation was identified. This case depicts awareness about this entity and use of appropriate immunostains, particularly the myoepithelial markers are essential to arrive at a correct diagnosis. Importantly, high-grade transformation, recurrence, and metastases are not very uncommon in EMC, warranting a correct and timely diagnosis for therapeutic decision-making and prognostication of the patient.