A 68-year-old man presented with weight loss (3kg in 5 months). His past medical history was remarkable for ART-controlled HIV infection (HIV viral load <20 and CD4+ lymphocyte count 692/uL). Review of systems revealed no other symptomatology. Chest X-ray showed a right-sided pleural effusion (Fig. 1).
Figure 1. Chest x-ray (posterior-anterior view):right-sided pleural effusion.
A diagnostic thoracentesis was then performed and revealed a very viscous yellow pleural fluid, which biochemical analysis was compatible with exudative effusion. Fluid cultures were negative. Cytological analysis of the pleural fluid showed morular papillary neoplastic cells with cytoplasmic vacuoles compatible with adenocarcinoma. However, cell block immunohistochemistry was positive for CK5/6 and negative for BerEP4, TTF1 and CDX2 favoring the diagnosis of mesothelioma. To assist the differential diagnosis a pleural biopsy was done but it was inconclusive due to insufficient material.
A contrast-enhanced thoracic computed tomography (CT) was then performed and revealed a large right-sided pleural effusion and a 16x10x5cm hypodense non-enhancing posterior mediastinal mass, anterior to the vertebrae and causing esophageal and aortic artery axis deviation, without invasion or pleural thickening; no parenchymal lung masses or lymphadenopathies were seen (Fig. 2).
Figure 2. Thoracic CT (a – coronal view; b – axial view): large right-sided pleural effusion and 16x10x5cm hypodense posterior mediastinal mass.
To further characterize this mediastinal mass an endoscopic ultrasound (EUS) was done and showed a very large well-defined rounded hypoechogenic mediastinal mass, located between 29 and 39cm from the incisors, adjacent to the esophagus and aorta, without invasion (Fig. 3a) This mass had heterogeneous echotexture, with some septa and some small interior cystic areas. EUS-fine needle aspiration (EUS-FNA) using a 25G needle with rapid on site cytological examination (ROSE) was performed (Fig. 3b).
Figure 3. EUS (transesophageal view): a – rounded hypoechogenic mediastinal mass adjacent to the esophagus and aorta; b – EUS-FNA.
Histopathological analysis demonstrated a neoplastic lesion with epithelioid cells having polygonal cytoplasm and round nuclei, which immunohistochemistry was positive for WT1 and calretinin (Fig. 4). The final diagnosis was mesothelioma.
Figure 4. Pathology: a – Cytology smears were markedly cellular, composed of morular and papillary neoplastic cells demonstrating slit-like intercellular windows (arrow) (Giemsa staining, 400x); b – Immunohistochemistry was positive for calretinin (200x).
The differential diagnosis of masses arising in the posterior mediastinum is extremely broad and includes lymphoma, pulmonary malignancies, granulomatous diseases, neurogenic tumors and cardiovascular lesions1. The posterior and middle mediastinum is relatively inaccessible to percutaneous approaches for tissue diagnosis because it is surrounded by spine posteriorly, lungs laterally and the trachea, heart, and great vessels anteriorly2. Therefore mediastinoscopy, thoracoscopic mediastinal biopsy, or transbronchial biopsy have historically been used, with morbidity, time, and costs limiting their applicability3.
Advancement in the clinical application and use of EUS in recent years has transformed the field of gastroenterology, with the ability to identify and manage a wide variety of disorders, even extending beyond the gastrointestinal tract. The esophagus affords an excellent acoustic window providing the direct visualization and characterization of several mediastinal structures and the guided sampling of tissue without the danger of traversing major blood vessels, para-spinal tissues, or the lungs2. EUS-guided biopsies allow immunostaining in about 80-90% cases which is of importance for subtyping of NSCLC, differential diagnosis to metastases and mesothelioma and for diagnosis of granulomatous diseases and lymphoma4. In addition, genotyping of adenocarcinoma, flow-cytometry, FISH analysis and other cytogenetic methods are possible using material obtained by EUS-FNA from mediastinal lesions4. ROSE significantly improved the rate of complete genotyping and reduced the need for additional needle passes and repeat invasive procedures aiming at molecular diagnosis4.
The evaluation of mediastinal masses remains a significant challenge for clinicians2. The efficient and precise management of these patients depends on accurate mediastinal imaging and on adequate tissue sampling2. EUS-FNA should be considered in all patients with suspicious posterior mediastinal lesions and in those patients with an indeterminate result on CT3.
Catarina Félix1, Susana Marques1, Martinha Chorão2, Sância Ramos2, Cristina Chagas1