Pulmonary mucoepidermoid carcinoma: Clinicopathological characteristics of 20 cases and a literature review

Background: Pulmonary mucoepidermoid carcinoma is a rare tumor of the lung. The clinicopathological characteristics of pulmonary mucoepidermoid carcinoma are not well defined due to the low incidence. This study was performed to provide more supplementary clues for the identification and understanding of pulmonary mucoepidermoid carcinoma. Methods: We reviewed the medical records since January 1, 2000 to December 31, 2018. The patients’ medical recordsincluding age at the time of diagnosis , gender, smoking history, preoperative evaluations, operative procedures, tumor location, tumor size, tumor stage, lymph node metastasis, pathological markers, prognosis and survival information were extracted and reviewed. Categorical variables were presented as parameters and percentages. A comparison was performed between patients with high and low grade of pulmonary mucoepidermoid carcinoma. Results: 20 patients were identified and the age span is from 18 to 67 year-old with the average age is 45. Mucoepidermoid carcinomas were commonly found in men60%. 80% patients had clinical presentations and the positive rate of tumor markers was 78%, although no specific tumor markers were found. TTF-1 were negative in all cases. ALK rearrangement was identified in a non-smoking woman with high grade pulmonary mucoepidermoid carcinoma. Surgery is the main procedure. 3-year survival rate is 72% and 80% patients achieved disease-free alive. High-grade patients tend to harbor older age (p=0.035), larger tumor volume (p=0.026) and higher index of ki-67(p=0.0005). Conclusions: Pulmonary mucoepidermoid carcinoma could occur in a wide age span. Early diagnosis and complete surgical resection may promise a good prognosis. Grading is a key factor to predict the overall survival time. Combined TTF-1 and MAML2 will benefit the identification of pulmonary mucoepidermoid carcinoma from other lung tumors. Future prospective randomized controlled trials and larger, multi-centric series are needed. preoperative evaluations, operative procedures, tumor location, tumor size, tumor stage, lymph node

presented as parameters and percentages. A comparison was performed between patients with high and low grade of pulmonary mucoepidermoid carcinoma. Results: 20 patients were identified and the age span is from 18 to 67 year-old with the average age is 45. Mucoepidermoid carcinomas were commonly found in men 60% . 80% patients had clinical presentations and the positive rate of tumor markers was 78%, although no specific tumor markers were found. TTF-1 were negative in all cases.
ALK rearrangement was identified in a non-smoking woman with high grade pulmonary mucoepidermoid carcinoma. Surgery is the main procedure. 3-year survival rate is 72% and 80% patients achieved disease-free alive. High-grade patients tend to harbor older age (p=0.035), larger tumor volume (p=0.026) and higher index of ki-67(p=0.0005). Conclusions: Pulmonary mucoepidermoid carcinoma could occur in a wide age span. Early diagnosis and complete surgical resection may promise a good prognosis. Grading is a key factor to predict the overall survival time.
Combined TTF-1 and MAML2 will benefit the identification of pulmonary mucoepidermoid carcinoma from other lung tumors. Future prospective randomized controlled trials and larger, multi-centric series are needed.

Background
Pulmonary mucoepidermoid carcinoma (PMEC) is a rare tumor that accounts for 0.1% to 0.2% of all pulmonary tumors. Most pulmonary MECs arise in the proximal bronchi [1]. Histologically, the classification of the PMEC is mainly based on the degree of mitosis necrosis and abnormalities of the 3 nucleus in the tumor. Low-grade mucoepidermoid carcinoma consists mainly of glandular-like structures and mucin-secreting cells, whereas high-grade PMECs include a large number of squamous cells [2]. However, in a few previous studies, intermediated grade had been suggested because of different grade systems described by Brandwein [3]. So far, most researches on PMEC belonged to either a series of cases report or small sample retrospective studies, especially in China [4][5][6]. The characteristics of PMEC are not well defined due to the low incidence of PMEC and there are a lot of controversies in the diagnosis and treatment of PMEC. In this paper, we analyzed the clinicopathological features, treatment and prognostic information of 20 PMEC patients who were exposed to 2000-2018 in Shanghai Ruijin Hospital and Shanghai Cancer center. This study was performed to provide more information for the identification and understanding of pulmonary mucoepidermoid carcinoma.

Patients and tissue specimens
The study was conducted under the approval by the Institutional Review Board. Ethical approval was granted by the clinical Research Ethics Committee of Shanghai Ruijin Hospital, Jiaotong University and Shanghai Cancer center, Fudan university. We reviewed the medical records, and outpatient pathology visits since January 1, 2000 to December 31, 2018 and collected 20 patients with PMEC, including 12 inpatients and 8 cases of pathology consultation patients whose data were partly missing. Signed informed consent was obtained from all induced patients for the acquisition and use of tissue samples and anonymized clinical data. Tissue specimens were collected from surgical resection or biopsy. The bronchoscopy was performed in 8 inpatients. Two inpatients had a diagnosis of PMEC by bronchoscopy, whereas the other patients were finally diagnosed through resected specimens. Hematoxylin and eosin(H&E) stained slides in all patients were reviewed by two experienced pathologists for histopathological confirmation. PMEC was divided into low or high grade according to the World Health Organization(WHO) criteria of lung cancer [7].

Data collection and extraction
The patients' medical records including age at the time of diagnosis , gender, smoking history, preoperative evaluations, operative procedures, tumor location, tumor size, tumor stage, lymph node metastasis, pathological markers, prognosis and survival information were extracted and reviewed.
Tumor stages were classified according to the American Joint Committee on Cancer Staging System, 7th edition. We acquired follow-up data from outpatient follow-ups or telephone and the last follow up time is December 31, 2018.

Statistical analysis
Categorical variables were presented as parameters and percentages. Nominal variables included gender, smoker, lymph node metastasis and TNM stage were conducted using Fisher's exact test.
Age, tumor size and Ki-67 labeling index were compared by way of an independent-samples t-test. All tests were two-sided, and p-values less than 0.05 were considered to be statistically significant.  Twenty patients were divided into low-grade (85%) and high-grade (15%) according to pathological diagnosis ( Figure 2). Low-grade PMECs show cystic patterns with solid areas, three cell types: bland columnar mucin-secreting cells, squamoid cells, and intermediate cells, with rare necrosis. While, high grade PMECs mainly show solid areas, which composed of atypical squamoid cells, accompanied by variable numbers of mucin-secreting cells, with frequent mitosis and necrosis. Some patients' pathological characteristics were available and summarized in Table2. The results showed that resection margins were negative in all 14 (100%) patients and no perineural invasion were found in 6(100%) cases who had been recorded. Intravascular embolus were found in 2(33%) cases. Pleural involvement were positive in 4(36%) patients among 11 patients. Bronchus invasion existed in 2 (21%)patients. TTF-1 were negative in all patients and P63 were detected in 67% patients.
Chemotherapies were performed in 3(25%)patients after operation among 12 cases and the patient accompanied with bone metastasis received local radiotherapy. 11 patients had follow-up records and the follow-up time ranged from 8 to 126 months (mean 52m). Two patients died of tumor at 8 and 18 months after diagnosis. Among these two patients, one did not undergo surgery because of bone metastasis and the other one suffered recurrence in ipsilateral lung after surgery. The remaining 9 patients were alive without evidence of recurrence or metastasis. A comparison was performed between patients with high and low grade of pulmonary mucoepidermoid carcinoma. There were statistically significant differences in the age(p=0.035), tumor size(p=0.026) and the index of ki-67(p=0.0005).
In clinic, the age range of PACC had been reported to be 21-80. PMEC had a broad spectrum in age from 3 to 78 years old and often occurred in 40s [5,10]. According to the study conducted by Kumar, PACC was mostly localized to the trachea and main bronchus, while PMEC was more commonly diagnosed in the lung. Patients with PMEC had a better prognosis than PACC. The age span of our series is from 18 to 67 year-old and the average age is 45. Interestingly, in accordance with recent reports, we also found that PMECs were commonly found in men [11]. 80% patients had clinical presentations and the positive rate of tumor markers was 78%, although no specific markers were found. Chest CT had an important value in the diagnosis of PMEC despite rare tumors exhibited an endobronchial pattern. If these patients were accompanied with persistent respiratory symptoms or elevated tumor markers, bronchoscopy examination and biopsy should be conducted to avoid misdiagnosis. TTF-1 was negative in all PMECs, conforming that it was of great value in distinguishing PMEC from lung adenocarcinoma, adenosquamous carcinoma and PMEC-like tumors [12]. Surgery is the main procedure of PMEC. Most patients were alive without evidence of disease after completely resection [13]. Our study showed that 3-year survival rate of PMEC is 72%, and 80 percent of those patients achieved disease-free alive. Histological grade was widely recognized as a prognostic factor.
Our series also demonstrated that high-grade PMEC patients tend to harbor older age, larger tumor volume and higher index of ki-67. Due to the small size of samples, we did not find significant difference in TNM stage and metastasis between low and high grade of PMEC. However, some large series had revealed the worse outcomes of patients who accompanied with advanced TNM staging and earlier metastasis [14][15][16]. At present, there were few studies focused on driven genes of PMEC. Evidences indicated that the most frequent gene mutation in PMEC is MAML2, which belongs to MasterMind-like gene family on chromosome 11q21. In salivary gland tumors, MAML2 rearrangement has been identified in 40-75% of mucoepidermoid carcinoma [17][18]. However, the association between MAML2 rearrangement and prognosis is still in suspense. The current view suggests that MALML2 is more prone to occur in lowergrade PMEC and has no value for prognosis [19][20][21]. PMEC has a low preoperative diagnosis rate through bronchoscopy and CT-guide percutaneous lung puncture biopsy. Most of our cases had been treated as non small cell lung cancer (NSCLC) before and during surgery. It suggested that the application of molecular techniques to detect the MAML2 rearrangement and TTF-1 expression may be helpful in improving the preoperative diagnosis. Targeted therapy has brought survival benefit to NSCLC patients. So far there is no effective treatment for PMEC patients who cannot undergo surgery.
Few studies had paid attention to driven genes such as EGFR and ALK. It was noted that the mutational frequency of EGFR in PMEC was 40% and mutations were all L858R mutation. A case of ALK-rearranged PMEC was reported by Wong et.al in 2009 [22][23][24]. In our retrospective cases, we identified ALK rearrangement in a non-smoking woman with high grade PMEC. However, due to the small scale of such report, no predictive markers have been found. More attention should be paid to investigate mutational profile of PMEC to provide alternative therapy for advanced patients, especillay MAML2 rearrangement.

Conclusions 8
Our study has provided supplementary clues for the identification of PMEC and updated the understanding of PMEC. Early diagnosis and complete surgical resection may promise a good prognosis. Combined TTF-1 and MAML2 will benefit the identification of pulmonary mucoepidermoid carcinoma from other lung tumors. Mutational profile of PMEC deserves further study. Considering that there is no effective treatment for cases accompanied with distant metastasis, driven genes of existing drugs such as EGFR and ALK were recommended to detect in clinic. However, as a retrospective study, it had inherent limitations due to the small size of sample. Meanwhile, some clinical data and follow-up information were not available. Future prospective randomized controlled trials and larger, multi-centric series are needed to overcome the challenges related to the accurate diagnosis and prediction of PMEC.  Local invasion included perineural invasion, blood vessel invasion, pleural involvement and bronchus invasion; Metastasis included lymph node, bone, abdominal and brain.