Pulmonary epithelial myoepithelial carcinoma with papillary architecture: an uncommon morphology of a rare tumour

Evolving Diagnostic Approach of Pulmonary Salivary Gland-type Tumors

Pulmonary salivary gland-type, although bear resemblance to their salivary gland counterparts, present a diagnostic challenge due to their rarity. Clinical features overlap with lung carcinoma; however, management strategies and outcomes are distinct. Onus falls on the pathologist to avoid misinterpretation of small biopsies especially in young, nonsmokers with slow growing or circumscribed endobronchial growths. A combination of cytokeratin, myoepithelial immunohistochemical markers, and identification of signature molecular alteration is invaluable in differentiation from lung cancers and subtyping the pulmonary salivary gland-type tumor.

Recent developments in the pathology of primary pulmonary salivary gland-type tumours

Primary pulmonary salivary gland-type tumours are rare neoplasms that are thought to arise from seromucinous glands that are located in the submucosa of large airways. These neoplasms have clinical and pathologic features that are distinct from other pulmonary neoplasms. The majority of primary pulmonary salivary gland-type tumours are malignant, with the most common entities being mucoepidermoid carcinoma, adenoid cystic carcinoma, and epithelial-myoepithelial carcinoma. Less commonly seen are myoepithelial carcinoma, hyalinizing clear cell carcinoma, acinic cell carcinoma, secretory carcinoma, salivary duct carcinoma, intraductal carcinoma, and polymorphous adenocarcinoma. Benign salivary gland-type tumours of the lung include pleomorphic adenoma and sialadenoma papilliferum. Morphologic, immunophenotypic, and molecular features of these neoplasms are largely similar to salivary gland tumours elsewhere, and therefore the exclusion of metastatic disease requires clinical and radiologic correlation. However, the differential diagnostic considerations are different in the lung. The distinction of salivary gland-type tumours from their histologic mimics is important for both prognostication and treatment decisions. Overall, salivary gland type-tumours tend to have a more favourable outcome than other pulmonary carcinomas, although high-grade variants exist for many of these tumour types. Recent advances in our understanding of the spectrum of salivary gland-type tumours reported in the lung and their diversity of molecular and immunohistochemical features have helped to refine the classification of these tumours and have highlighted a few differences between salivary gland-type tumours of the lung and those primary to other sites.

Pulmonary Epithelial-Myoepithelial Carcinoma

Pulmonary epithelial-myoepithelial carcinoma (P-EMC) is an exceptionally rare subtype of salivary gland lung tumor originating from tracheobronchial glands. P-EMC is a biphasic tumor consisting of an inner layer of epithelial cells and an outer layer of spindle-shaped, clear-cell-like myoepithelial cells. Bronchial obstruction symptom is the main clinical characteristic for P-EMC. Because its clinical and imaging characteristics are highly similar to other types of non-small-cell lung cancer (NSCLC), it is easy to cause missed diagnosis and misdiagnosis. The diagnosis is mainly based on the pathology and immunohistochemistry with an inner layer of epithelial cells immunoreactive for cytokeratin and an outside layer of myoepithelial cells immunoreactive for S100 protein (S-100) and smooth muscle actin (SMA). Therefore, positive for cytokeratin, S-100 and SMA can assist in the diagnosis. Although in general, P-EMC is a low-grade malignant neoplasm, it may occasionally recur and metastasize. The optimal method for P-EMC treatment has not been established, and surgical resection is still the main clinical method. Radiotherapy and chemotherapy have been shown not sensitive for P-EMC treatment, whereas targeted therapy and immunotherapy have not evaluated in clinical practice. This review focuses on the pathological characteristics, molecular characteristics, diagnosis, treatment, and prognosis of P-EMC.

Pulmonary epithelial-myoepithelial carcinoma (P-EMC) with focal high grade transformation: Molecular and cytologic findings

BACKGROUND

Pulmonary epithelial-myoepithelial carcinoma (P-EMC) is a rare type of salivary gland tumor of the lung. Diagnosis from preoperative biopsies or fine needle aspiration (FNA) cytology specimens is difficult given the rarity of the tumor and overlapping cytomorphology with other entities. These tumors generally have a good prognosis, however prior reports of recurrence and metastasis to lymph nodes have been reported. Further, little is known about the malignant potential of high grade transformation. The molecular characteristics of this entity are unknown, with only a few case reports commenting on molecular findings. Here, we report a case of P-EMC with focal high grade transformation. We present its diagnostic pitfalls on cytology specimens, surgical pathology, immunohistochemistry, and molecular findings.

CASE PRESENTATION

A 72 year old female presented with an incidentally detected lung mass. A chest computed tomography (CT) demonstrated a left hilar mass measuring 4.1 cm with endobronchial extension into the left upper lobe. On the initial endobronchial core needle biopsy and cytology FNA, the tumor was misinterpreted as squamous cell carcinoma. The patient subsequently underwent a left pneumonectomy along with mediastinal lymph node dissection. Final surgical pathology of the resection specimen indicated a P-EMC with focal high grade transformation. The patient is disease-free 1 year post-surgery.

CONCLUSIONS

Due to the rarity of P-EMC, insufficient sampling and histologic heterogeneity, diagnosis of P-EMC on preoperative core needle biopsy or FNA specimen is difficult. Herein, we present a rare case of P-EMC, with a pre-operative FNA cytology specimen that consisted of tumor cells with dense cytoplasm and moderate cytologic atypia, strong positive staining pattern of p40, that was misdiagnosed as squamous cell carcinoma. Follow up surgical resection showed P-EMC with focal high grade transformation. Salivary gland EMCs with high grade transformation have previously been reported to have a worse prognosis, however, little is known about the malignant potential in the lung. Next generation sequencing (NGS) using a 397-gene solid tumor panel identified variants in DNMT3A, APC, STAT3 in both low and high grade components, while KDM5C was present only in the high grade transformation.

Recent updates in salivary gland tumors of the lung

Salivary gland tumors are uncommon primary lesions in the lung. Their morphologic, immunophenotypic, and molecular characteristics resemble those of their counterparts in the head and neck or elsewhere. Most common primary pulmonary salivary gland tumors include mucoepidermoid carcinoma, adenoid cystic carcinoma, and epithelial-myoepithelial carcinoma. The study of these neoplasms is hampered by their paucity. Therefore, studies are in general small or restricted to individual cases. Despite this challenge recent advances have been made specifically at the molecular level. Molecular alterations such as MAML2 rearrangements in mucoepidermoid carcinoma, MYB rearrangements in adenoid cystic carcinomas, and EWSR1 rearrangements in hyalinizing clear cell carcinomas and myoepithelial tumors have been identified. These molecular alterations might be helpful in the distinction of these salivary gland tumors from other neoplasms in the lung. However, the distinction from metastatic disease remains challenging. Awareness of these tumors and knowledge of available ancillary studies to confirm the diagnosis is important to avoid misdiagnosis which might lead to differences in treatment, management, and prognosis. Further studies are needed to identify biomarkers to better predict patient's outcome and for individual management and treatment of patients.