Challenges in diagnosis of pulmonary mucoepidermoid carcinoma

Abstract Rationale: Pulmonary mucoepidermoid carcinomas (PMECs) of the lung are rare malignant tumors. Despite progresses in examinations, the tumor represents a diagnostic challenge for pathologists and clinical physicians. Here, we present a patient who was eventually diagnosed with PMEC by the bronchoscopic examinations conducted three times. Patient Concerns: We present the case of a 41-year-old female who was initially diagnosed with pulmonary pleomorphic adenoma (PPA) with a 68 × 82 mm mass and nodules in her lung and eventually diagnosed with PMEC. Diagnoses: Based on histopathology, immunohistology, and imaging studies, the patient was diagnosed with PMEC (pT4N2M1). Interventions: The patient received first-line systemic chemotherapy regime (gemcitabine combined with carboplatin). Outcomes: The patient received 2 cycles of chemotherapy. Based on the response evaluation criteria in solid tumor, she achieved partial response, and the mass was distinctly decreased from 68 × 22 mm to 41 × 17 mm. Lessons: This case presents a rare PMEC overlapping with PPA, based on histological findings, suggesting that besides imaging studies and laboratory examinations, multiple biopsies and ThinPrep cytology tests are necessary to obtain an accurate diagnosis. The patient showed positive response to chemotherapy.


Introduction
Pulmonary mucoepidermoid carcinoma (PMEC) is a rare neoplasm that accounts for 0.1% to 0.2% of all malignant lung tumors. [1] PMEC is characterized by squamous cells, mucussecreting cells, and intermediate cells. [2,3] Pulmonary pleomorphic adenoma (PPA) is a rare benign tumor with epithelial and myoepithelial cells and a mesenchymal component that could be chrondromyxoid stroma. PPA and PMEC are both solid tumors with pleomorphic histological appearances. The similarity of histological components on biopsy increases the difficulty of diagnosis. Here, we present the case of a rare PMEC that was distinguished from PPA by pathological examinations after three bronchoscopic biopsies and a ThinPrep cytology test (TCT). The patient benefited well from chemotherapy, and we hope that this report can provide useful evidences in the diagnosis and treatment strategy of PMEC.

Case presentation
An informed written consent was obtained from the patient for the publication of this case report and accompanying images. A 41-year-old female was admitted to our hospital with progressive cough and fever for 6 months and 10 days, respectively, in March 2017. The patient had dry cough, a maximum body temperature of 39.4°C, no respiratory disease, and physical examination suggested no significant abnormality.
A routine laboratory examination indicated that neuronspecific enolase (NSE), cytokeratin19 fragment antigen, and carcinoembryonic antigen 12-5 were slightly high. Contrast enhanced computed tomography (CECT) scan revealed a wellcircumscribed mass of soft tissue in the posterior segment of the left lung, near the hilum, with the involvement of dorsal segment of the lower lobe. The mass was approximately 68 Â 82 mm in size and connected to the pleura (Fig. 1A). A lowdensity necrotic lesion in the mass and inhomogeneous enhancement was observed (Fig. 1B). Soft tissue nodules (∼11 Â 6 mm) in the left upper lobe and apex of the right lung, and swollen lymph nodes in the mediastinum were observed ( Fig. 1C and D). In addition, the positron emission tomography computed tomography (PET-CT) scan revealed metabolic activity indicative of a mass near the left hilum, and other entities in the lung also showed high 18 F-fluorodeoxyglucose uptake. The maximum standardized uptake value (SUVmax) of the patient was 10.72 (Fig. 1F). Therefore, the initial diagnosis upon admission was lung cancer with intrapulmonary and mediastinal metastasis. The first bronchoscopic biopsy revealed large numbers of necrotic lesions and few atypical cells visible under the microscope ( Fig. 2A). However, the results had no diagnostic value. The second bronchoscopic biopsy revealed epithelial-like cells that formed a cord-like pattern and large numbers of intracellular stroma visible under the microscope (Fig. 2B). No pathologic mitotic abnormality and mucin-secreting cells were observed. Immunohistochemistry revealed that the material was positive for cytokeratin (CK), epithelial membrane antigen (EMA), P63, cytokeratin 7 (CK7), and soluble protein-100 and negative for vimentin (VIM), cytokeratin 20 (CK20), synaptophysin (Syn), thyroidtranscription factor-1 (TTF-1), carcino-embryonic antigen (CEA), NSE, and calponin. Based on the histology and immunhistochemistry results, the second biopsy revealed a diagnosis of PPA. However, PPA is a benign pulmonary adenoma and shows no aggressive biological behavior, including malignant transformation. [2] The diagnosis of PPA does not explain imaging behaviors, such as nodules in the lung and swollen lymph nodes in the mediastinum and the malignant cells that existed in the first biopsy. With the patient's positive cooperation, the third bronchoscopic biopsy was performed. Epithelial-like cells, mucus matrix, and some atypical cells of epithelial origin were observed in the third tissue specimen (Fig. 2C). Pathology revealed a low-grade variants MEC. The immunohistochemical analysis showed positive staining for P63, CK7, EMA, and cytokeratin 5/6 (CK5/6). TCT was undertaken in the third bronchoscopy, and the specimen revealed plenty of atypical cells, immunostaining was positive for CK, VIM, and EMA and negative for TTF-1, Syn, CEA, and Napsina. The third biopsies and TCT results exhibited atypical cells from the epithelium, and the biopsies were positive for MEC-specific molecular markers-P63, CK7, EMA, and CK5/6. [4] Finally, the patient was diagnosed with low-grade pulmonary mucoepidermoid carcinoma (pT4N2M1).
According to the first-line chemotherapy for advanced nonsmall cell lung cancer (NSCLC), the patient received gemcitabine (1000 mg/m 2 on day 1 and day 8) and carboplatin (area under the curve = 5) on day 1 every 3 weeks for 4 cycles. The patient completed 2 cycles of therapy. The mass in the left lung decreased from 68 Â 22 mm to 41 Â 17 mm (Fig. 1E). According to the response evaluation criteria in solid tumors (RECIST), the patient achieved partial response (PR). The patient showed no emerging recurrence or metastasis after a follow-up of 18 months. Adverse events were evaluated according to the common terminology criteria from adverse events (CTCAE), she was absence of symptoms, and the toxicity grade of the patient was grade 1.

Discussion
PMEC belongs to the salivary-gland type of lung malignant neoplasms and arises from the salivary gland-type epithelium of the central airway. [5] Histologically, PMEC consists of epithelioid cells (or squamoid cells), mucin-secreting cells, and intermediate cells (or transitional cells) that are divided into high-grade and low-grade variants. These cells behave in various growth architectures, including nest, papillary, or island structures. [6] Foci of calcification and ossification occurs occasionally. [7] PMEC can occur at any age, and most of the patients with PMEC presented with a large bronchial obstruction with nonproductive coughs, chest pain, or post obstructive pneumonia. [8] Some patients were asymptomatic, and symptoms were noted only during routine health examinations. [9] PPA is a biphasic neoplasm with epithelial or myoepithelial cells and a mesenchymal component, including mucoid or chondromyxoid stroma. [10] Epithelial cells and intracellular stroma were observed in patient's second biopsy. Myoepithelial participation was also reported in PMEC in 1986. [11,12] The overlapping of histopathological features in the 2 mixed tumors poses a diagnostic challenge for pathologists. In our case, the second biopsy results supported the diagnosis of PPA. However, microscopic examinations of the first and third biopsies found www.md-journal.com malignant cells, CECT scan images showed intrapulmonary and mediastinum metastasis, and PET-CT revealed a high uptake tumor. Overall, these results were suggestive of a malignant tumor. The immunohistochemistry studies also proved that the tumor arised from the epithelium. According to the literature available on PPA and PMEC, hyperplasia of the epithelium in PPA might lead to the transformation of PPA into PMEC. [2,6] The patient's immunohistochemistry results showed that the MECspecific molecular markers P63, CK7, EMA, and CK5/6 were positive. We confirmed the final diagnosis of this case was PMEC not PPA. Smetanan initially described PMEC as originating from the bronchus in 1952, [12] and since then, a few cases have been described further. Besides the relative rarity, the application of bronchoscopic biopsy also increases the difficulty for pathologists to obtain an accurate diagnosis. [13,14] Therefore, surgery was the first consideration for the diagnosis of PMEC. If the patients could not undergo an operation, multiple biopsies through a bronchoscope or CT-guided percutaneous were recommended. PMEC originates from the submucous myoma and are well encapsulated; and in our case, this probably explains why smear tests performed thrice showed no signs of tumor cells. However, TCT revealed plenty of atypical cells. Hence, TCT is helpful for the diagnosis of PMEC.
The routine treatment of PMEC is surgical resection. Sleeve lobectomy is frequent performed to remove the complete tumor with nodal dissection. [9] There is no sufficient evidence to prove that adjuvant chemotherapy or radiotherapy is necessary for a patient with complete tumor resection. [15] The CRTCl-MAML2 fusion gene generates the CRTC1-MAML2 fusion protein. [16] This novel protein not only leads to up-regulation of the EGFR ligand amphiregulin but also contributes to tumor development by interfering with cell growth regulatory pathways. [15,17,18] The CRTC1-MAML2 fusion gene is a possible new therapeutic target for PMEC. A few cases showed that patients with EGFR gene mutations benefit from treatment with TKI. [19,20] However, patients without EGFR mutation also benefited from TKI treatments. [21] Thus, the relationship between EGFR expression and TKI treatment warrants further research. However, TKI treatment was not opted for our patient because she refused genetic testing. Because of the metastasis of the malignant tumor, the patient received chemotherapy, and the regime was gemcitabine combined with carboplatin. After two cycles of chemotherapy, the mass in the left lung distinctly decreased from 68 Â 22 mm to 41 Â 17 mm and showed no emerging recurrence or metastasis from April 2017 to September 2018. According to the RECIST, the patient achieved PR.
Important prognostic factors of PMEC include histological typing, TNM stage, radioactivity uptake, and age. [8] As opposed to high-grade PMEC, the prognosis of low-grade PMEC is excellent. Although the patient histological typing is low-grade. Unfortunately, she lost the chance of operation, based on intrapulmonary and mediastinal metastasis in the CECT scan images, the TNM stage of the patient was pT4N2M1. So, we infer our patient has poor prognosis. Besides, radioactivity uptake of PET-CT scan indicated that patients with an SUVmax > 6.5 were thought to likely have a high-uptake tumor, [22,23] the SUVmax of the patient was 10.72. therefore, it was a high-uptake tumor. And high SUVmax in PET-CT had higher tumor grade, more frequent lymph node metastasis, and a worse long-term outcome. [22] The prognosis is expected to be better when discovring and starting treatment early.

Conclusion
PMEC is a primary malignancy of the lung that may be microscopically disguised as PPA. In diagnostically challenging cases, multiple pathological biopsies and TCT immunohistochemical staining are necessary, besides other ancillary examinations like image studies and laboratory examination.