A Literature Review of Localized Mediastinal Mesothelioma and a Rare Case Report

Mediastinal mesothelioma, mainly malignant pleural mesothelioma (MPM), is rare among primary cardiac and pericardial tumors, and its incidence ranges from 0.0017% to 0.33% in case reports or collected autopsies [1]. Pericardial MPM is a rare and fatal pericardial mesothelial surface disease, accounting for 0.7% of all mesothelioma cases, with an incidence of 1 in 40 million; in 500,000 autopsies, it has an incidence of 0.0022% [2]. MPM may be present in the pleura (70 75%), rarely in the peritoneum (20 25%), pericardium (4%), or rarely elsewhere, and the incidence of localized mediastinal mesothelioma is even lower, which has only been reported in a single case [3-4]. In this report, we described the clinical presentation, imaging findings, intraoperative findings, histopathology, and immunohistochemistry of an incidentally discovered localized mesothelioma of the Abstract


Introduction
had no obvious history of asbestos exposure. Physical examination revealed no abnormality. Coronary CTA incidentally revealed a tumor in the middle and posterior upper mediastinum (Table 1).
Imaging findings: An anterior chest radiograph done 8 months revealed no abnormalities ( Figure 1). The two-dimensional B-ultrasound image showed a mild hypoechoic mass in the upper mediastinum with clear boundaries, oval shape, uneven internal echo and no change in tail echo. Color Doppler flow imaging showed no obvious blood flow signal ( Figure 2). Plain scan and contrast-enhanced CT scan showed a 43 mm × 36 mm × 36 mm tumor in the middle and posterior superior mediastinum, with a CT value of about 41.5 HU, an arterial phase CT value of about 59.8HU and a venous phase CT value of about 77.5HU during the contrastcontrast-enhanced scan, with heterogeneous moderate enhancement; the tumor was adjacent to the ascending aorta, trachea, superior vena cava and right pulmonary artery. It was closely connected to the left chest wall, abutting the aortic wall, with a rough margin and indistinct demarcation from the surroundings, and no pleural or pericardial effusion was seen (Figuer 3).
Multidisciplinary consultation in our hospital, suspected mediastinal malignancy, and detailed systemic examination revealed no other metastatic or primary lesion, and we performed a thoracotomy. Intraoperative findings: There was a mass above the aortic arch of the posterosuperior mediastinum and in the left rear of the esophagus (about T3 -5 level), which abutted the esophagus and aortic arch and was tightly attached to the left chest wall, and the size of the mass was about 30 mm × 25 mm × 60 mm, which was hard and had poor range of motion.
Most of the tissues were cystic wall-like, with fibrous collagenization of the capsule wall accompanied by more lymphocyte infiltration; atypical cells showed papillary, patchy, pseudo glandular, and infiltrative growth in the focal part near the surface area; tumor cells were polymorphic, mainly shuttle-shaped and oval, with abundant red staining of the cytoplasm, irregular karyotype, and visible mitoses (   Primary MPM of the pericardium accounts for less than 5% of diagnosed mesotheliomas [9]. It usually presents as multiple pericardial nodules, with or without a dominant mass, or a diffuse tumor, encasing at least a portion of the heart. There are three types of reports: extrapericardial disease (10%); localized pericardial disease (15%); diffuse nodular tumors, which can invade the heart (75%) [10][11]. Localized (solitary) pericardial mesothelioma is considered a rare variant of MPM. In 2002, J.Fernando et al. [12] reviewed only 4 cases in the literature, and S.Cao et al. [13] reported 8 cases in 2017, an increase of 4 cases compared with before. Marchevsky et al. [14] reviewed 101 cases of localized mesothelioma reported in the English literature before July 2019, which did not show an increase in the number of cases of mediastinal mesothelioma. We reviewed the English literature before May 2020, and found only 8 cases of localized (solitary) mesothelioma, with the reported cases shown in Table 2. Together with our case, there were a total of 9 cases, our case was mediastinal pleural mesothelioma, and the rest were pericardial mesothelioma. Among the 9 cases, there was only one patient with a history of predisposing

Follow-up and Outcomes
There has been significant progress in the treatment of malignant mesothelioma, although it remains extremely difficult to treat.

Discussion
MPM is a rare malignant tumor that is invasive and generally produces mesothelial cells such as pericardium, pleura, and peritoneum. Thoracic MPM is the most common, accounting for approximately 70% -80% in terms of the total incidence of MPM, followed by malignant peritoneal mesothelioma [5]. MPM is characterized by a long latency period, a high degree of malignancy, and a poor prognosis. The pathogenesis of the disease is still unclear. At present, asbestos exposure is considered to be an important predisposing factor, and tuberculosis, radiation, thorotrast, etc. are also one of the predisposing factors [6][7][8]. Now more and more reported patients do not have a confirmed history of asbestos exposure. The main clinical symptoms are cough, chest pain, chest tightness, dyspnea, etc., and the clinical manifestations have no specificity, which brings great factors, all of which were malignant tumors, involving 4 females and 5 males. We analyzed 3 patients with only CT scan data and 1 patient with only MRI contrastcontrast-enhanced scan data, and 1 patient with CT contrast-enhanced scan data and MRI contrast-enhanced scan data. In 4 articles, there were only CT images or MRI images,The CT image findings were not described in the article. Therefore, two senior chest attending physicians in our hospital analyzed the CT image or MRI image in the literature and our case, respectively, and obtained the image findings after discussion following listening to different views. Results showed that the image findings showed 4 cases of localized soft tissue mass, and 1 case of cystic solid lesion (cystic dominantly), after 3 months showed recurrent mass at the same site; the soft tissue density and recurrent mass was basically the same as that of muscle, 3 patients had pericardial effusion. After contrast-enhanced scan, the soft tissue CT findings showed 5 cases of mild enhancement (solid part in cystic solid lesion and recurrent mass), and 1 case of moderate enhancement; the signal after MRI T1WI weighted fat sequence enhancement was basically the same as that of muscle ( Table 2). The CT findings of localized mediastinal mesothelioma were basically the same as those localized abdominal mesothelioma described in the article of Liu, Y.et al [15] .
In conclusions, mediastinal mesothelioma is characterized by unclear etiology, no specific manifestations, high misdiagnosis rate and poor prognosis. Pericardial mesothelioma is remarkable. Early diagnosis is beneficial to the treatment of patients. The imaging findings of localized mesothelioma in the mediastinum have not been summarized in the previous literature. This paper presents a summary. Although the number of cases is small, it has certain imaging findings. The case suggests that when the pericardium has circumscribed thickening, the mediastinal tumor is closely connected to the pleura, with the manifestation of soft tissue density, mild-moderate enhancement after contrast-enhanced scan, and pericardial effusion; the possibility of mediastinal mesothelioma should be considered.

Patient Perspective
The patient shared ideas to decrease their burden of treatment and very satisfied with the treatment.

Methods
Formal approval was granted by the Zhuhai Hospital of Guangdong Hospital of traditional Chinese Medicine.

Consent
Written informed consent was obtained form  Abbreviations MPM, malignant pleural mesothelioma; CT, Computed tomography; MRI, Magnetic resonance imaging; CTCA, computer tomography Coronary artery

Availability of supporting data
The datasets used or analysed during the current study are available from the corresponding author on reasonable request.