Primary Epithelial Myoepithelial Carcinoma of Lung, Reporting of a Rare Entity, Its Molecular Histogenesis and Review of the Literature

Primary epithelial myoepithelial carcinoma of lung is a rare entity and is thought to arise from the submucosal bronchial glands distributed throughout the lower respiratory tract. Because of the rarity of this tumor, we describe one case of epithelial myoepithelial carcinoma arising in the bronchus intermedius and presenting as an endobronchial mass. A 57-year-old male patient presented with an incidental finding of an endobronchial mass located in the lumen of the right lower lobe bronchus and caused near total luminal occlusion of the bronchus. An endobronchial carcinoid tumor was entertained clinically. Subsequently the patient underwent an uneventful videothoracoscopic lobectomy of lower and middle lobes of the right lung. Morphologically and immunohistochemically the tumor was characterized by two cell populations with epithelial and myoepithelial cells forming duct-like structure. The final diagnosis of epithelial myoepithelial carcinoma of lung was rendered.


Case Presentation
57-year-old male patient presented with colicky abdominal pain located at the previous incisional hernia site of the anterior abdominal wall. He had a history of smoking 1 cigarette pack per day for 20 years but quit over 10 years ago. His work up included a CT scan of the abdomen including slices from the lower chest. There was an incidental finding of an endobronchial mass of 2.0 centimeters in diameter and the mass almost entirely occluded the lumen of the right lower lobe bronchus. Patient denied any history of cough, shortness of breath, hemoptysis, fever, or weight loss. A PET/CT scan showed low-grade uptake with an SUV of 1.8 in the mass (Figure 1). Fibreoptic bronchoscopy showed a friable, smooth, and highly vascular looking mass in the right lower bronchus. Clinical diagnosis was endobronchial carcinoid tumor. The patient denied symptoms of a carcinoid syndrome, such as, flushing, diarrhea, wheezing, and cyanosis. Subsequently he underwent an uneventful videothoracoscopic right lower and middle lobectomy.

Materials and Methods
The tissue was fixed in 10% buffered formalin for 24 hours and embedded in paraffin. Sections were taken and stained with hematoxylin eosin. The immunohistochemical stains were performed at an outside facility, using the Leica polymer detection system Intense on the BondMax staining platform. The following antibodies and proteins were used: Cytokeratins AE1/AE3 (Invitrogen, Frederick MD; 1 : 400), CAM5.

Pathology
Grossly, the tumor was a solitary, well-circumscribed, whitetan, polypoid, and unencapsulated mass located in the bronchus intermedius. It measured 1.2 × 1.0 × 0.8 cm in greatest dimension ( Figure 2). Cut surface was solid whitish tan with no evidence of necrosis or hemorrhage. Microscopically, the mass was situated beneath the bronchial epithelium without capsule. The mass was composed of duct like structures, lined by two distinct cell layers. The inner cell layer was cuboidal cells with eosinophilic cytoplasm and centrally located nuclei and outer layer of cells with predominantly clear cytoplasm and uniform nuclei. The ducts like structures contained eosinophilic material in the luminal spaces. Some areas of the tumor showed more spindly cells with clear cytoplasm (Figures 3 and 4). The tumor showed 2-3 mitosis/10 high-power field (HPF). Immunohistochemical studies confirmed the biphasic nature of the tumor. The inner layer of ducts was reactive for cytokeratins (CAM5.2, AE1/AE3, CK7 and CK903) indicating epithelial cells differentiation, and outer layer of cells was reactive for S-100, calponin, SMA, GFAP suggestive of myoepithelial cells. CD117 showed positive reactivity in ducts and spindle cells (Figures 5,6,7,8,and 9). The tumor did not show reactivity for synaptophysin, chromogranin, thyroglobulin, TTF-1 and CK20, BerEP4, B72.3, and PSA. Ki-67 showed 2-3% of proliferative activity ( Figure 10). Immunostain for p27/kip-1 was performed and it showed nuclear reactivity in the epithelial component and minimal cytoplasmic positivity in the myoepithelial component (Table 2).

Discussion
Primary epithelial myoepithelial tumors of lung are rare neoplasms. These tumors are considered to be low-grade malignant neoplasms with histologic features similar to their counterparts of the salivary gland. Neoplasms with similar morphology are reported in the breast, skin, and lacrimal glands [12][13][14]. It is a relatively less common entity that had only been recognized within the last 2 decades [12]. The WHO defines this entity as "a malignant tumor composed of variable proportions of two-cell types, which typically form duct like structures. The biphasic morphology is represented by an inner layer of duct lining, epithelial type cells, and an outer layer of clear myoepithelial cells" [15]. To date, 24 cases have been reported in the lung, variably classified as "adenomyoepithelioma", "pneumocytic adenomyoepithelioma", "myoepithelioma", "epithelial myoepithelial carcinoma", and epithelial myoepithelial tumor. One suggested term by Pelosi was pulmonary epithelial myoepithelial tumor of unproven malignant potential (PEMTUMP) [7,12].
A review of the literature reveals a total of 25 cases including the current case in the last 18 years. Out of 25 cases, there is slightly female predominance, 15 are females and 10 are males. Age range is 34-76 years and the average age is 54 years. 13 cases arise on the right side, 11 arise on left side, and one case does not state the site. The size of the tumor ranges from 0.8 cm to 16 cm; with average of 3.2 cm. Sizes are not available in two cases. Most of the cases are presented as polypoid endobronchial mass. Grossly these tumors are not encapsulated but well-delineated masses, and they present as an exophytic intrabronchial growth pattern, sometimes completely obstructing the bronchial lumina [13]. One recent case is described as being intraparenchymatous mass without apparent bronchial or visceral pleura connection [13]. All cases show characteristic morphology with biphasic pattern of an inner layer of epithelial cells immunoreactive for cytokeratins and an outer layer of myoepithelial cells immunoreactive for S-100 and smooth muscle actin (SMA).  of duct-like structures. Necrosis is rare, but it has been described [15]. Ki-67 labeling is reported in few cases and it ranges from <1% to 12%. Since there are only few cases reported in the literature with limited followup, the overall approach is to complete excision of these tumors because of infiltrating morphologic pattern. So far there is only one case that showed extensive lymphovascular, perineural invasion, and lymph node involvement and local metastasis [12]. Followup of 22 out of 25 cases ranges from 4 months to 7 years. No followup is available in three cases including one case lost to followup. The rest of cases reported no recurrence or distant metastasis ( Table 1). The disease-free survival and lack of nodal involvement in majority of cases correlate well with the lack of recognized histopathologic features of aggressiveness [8].
The biologic behavior of this type of tumor is still unclear. Attempts were made by Pelosi et al. to find out the changes at molecular level and further supported by Munoz et al. [13] Protein p27/kip-1 is a cyclin-dependent kinase inhibitor (CDK) that blocks cell cycle in G0 and G1. It is present in high concentration in quiescent cells and its levels slowly decrease while cells are stimulated to begin the cell cycle. Thus, p27/kip-1 inhibits and controls the progression of the cell cycle and therefore exercises a function of inhibition of tumorigenesis; in fact, it has been demonstrated that levels of p27/kip-1 are decreased in many tumors. Moreover, Besson et al. have recently described a dual function of this protein; as well as acting as an inhibitor of tumorigenesis, it would have oncogenic functions when it presents cytoplasmic location, acting through mediators which are little known [16]. The work also suggests that p27/kip-1 oncogenic activity leads to aberrant stem and progenitor cell expansion in the lung and retina. This study provides the first direct in vivo evidence that in addition to its role as a tumor suppressor, p27/kip-1 also functions as an oncogene. So, in epithelial-myoepithelial tumors of lung, in accordance with Pelosi et al. [7], an aberrant subcellular location of p27/kip-1 into the myoepithelial cell would provoke the loss of its growth-inhibition function through the lack of restriction of proliferation of myoepithelial component. Munoz et al. support this concept along with proved oncogenic function of this protein [13]. In our case, nuclear epithelial positivity is more pronounced. There is minimal cytoplasmic positivity seen in the myoepithelial component which verifies this concept, but evaluation of more cases is needed to support this concept. In conclusion, epithelial-myoepithelial tumor is a neoplasia of uncertain malignant potential, which can arise exceptionally in the lung. Epithelial myoepithelial carcinoma is not difficult to diagnose in resection specimen because of characteristic morphology and immunohistochemical studies. Only in very scant biopsy material or sampling of areas without the characteristic bilayered tubules may cause difficulty and confusion with other neoplasms [12].
The differential diagnosis can be extensive and in many occasions it is going to depend on the relative predominance of myoepithelial component or on the biphasic pattern. If the tumor has predominantly biphasic pattern, the differential diagnosis includes pulmonary pleomorphic adenoma. If there is predominance of clear cells (myoepithelial cells) then myoepithelioma, myoepithelial carcinoma, and clear cell "sugar" tumor are in differential diagnosis. Clear cell "sugar" tumor can be differentiated on immunohistochemical stains since these tumors show positivity for melanocytic markers like HMB-45 and negativity for cytokeratins. If a tumor shows infiltrating pattern with some solid areas, mucoepidermoid carcinoma, acinic cell carcinoma, adenoid cystic carcinoma, and metastatic renal cell carcinoma are other differentials to consider. Surgical resection appears to be the treatment of choice by any of the current surgical modalities and depending on the clinical setting.