Mediastinal neurogenic tumors most often arise in the posterior compartment of the mediastinum and are classified by the cell type of origin. A correct diagnosis is of utmost importance as benign neurogenic tumors have an excellent prognosis, whereas malignant neurogenic tumors have a poor outcome. A mediastinal peripheral nerve sheath tumor diagnosed by endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) has, to our knowledge, never been reported before. In this report, we describe our diagnostic approach in a 41-year-old woman presenting with a paratracheal mass on a chest x-ray.
A 41-year-old woman, with known nicotine abuse and gastroesophageal reflux, presented at an outpatient clinic with chronic cough. A chest x-ray revealed a dense mass in the right paratracheal area of the superior mediastinum. A computerized tomography (CT) scan revealed a homogenous mass of 31×29×42 mm, deviating the trachea to the left without invading its wall and mildly enhancing on intravenously administered iodine contrast (Fig. 1). The solid nature of this lesion was proven on chest magnetic resonance imaging, revealing increased vascularization and signs of focal necrosis, white spots on T2-weighted images, and black spots on T1-weighted images (Fig. 2). The paratracheal mass showed a high fluoro-deoxy-glucose uptake on positron emission tomography scan, without any evidence of distant spread. The mass was easily visualized using EBUS, and a TBNA was performed. Pathologic examination of the cell block preparation revealed an increased cellularity of bundled, spindle-shaped cells in a fascicular arrangement with regional palissading. Cell nuclei were oval and hyperchromatic, with scarce mitosis. Immunocytochemistry of these cells showed a positive S-100 staining but absence of melanine A, HMB45, c-kit, desmin, α-smooth muscle actin, CD34, epithelial membrane antigen, prekeratine, and tyrosinase stainings, revealing a schwannian nature for this lesion. Given the cytologic nuclear features (oval, hyperchromatic) and scarce mitoses, in combination with CT and metabolic information, we were not able to differentiate between a benign schwannoma and a malignant peripheral nerve sheath tumor (MPNST). A subsequent video-assisted thoracoscopy was performed with total excision of the soft paratracheal mass (weighing 21 g) originating from the vagal nerve, distally from the recurrent laryngeal nerve. The histology of the surgical resection specimen was compatible with a benign cellular schwannoma of the vagal nerve sheath, characterized by encapsulation, inflammatory foci, hyaline vessel walls, and diffuse S-100 staining (Fig. 3).
Both benign and MPNSTs represent approximately 10% of all soft tissue neoplasms.1 Benign peripheral nerve sheath tumors (BPNSTs) are subdivided into 3 groups: schwannoma, neurofibroma, and perineuroma. MPNST is a rare variant of soft tissue sarcomas. Schwann cells are the major contributors to the formation of a benign (ie, schwannoma) and a malignant (ie spindle cell sarcoma) neoplasm of the nerve sheath.2 A schwannoma occurs between the second and the fifth decades, and most frequently involves the head and neck area (cervical plexus), the flexor aspects of the limbs (peroneal and ulnar nerves), and are less often situated in the posterior mediastinum.1 A true capsule consisting of the epineurium surrounds the BPNST as they arise within the nerve sheath. Most BPNSTs are slow growing, solitary, and accidently discovered as neurological symptoms or pain are uncommon unless the tumor, due to its enlarging size, exerts pressure on neighboring structures. MPNST also present between the second and the fifth decades and most commonly involve the major nerves including the sciatic nerve, brachial plexus, and sacral plexus. Patients with a MPNST may present with pain or neurological symptoms due to neural invasion. Moreover, an association with neurofibromatosis type 1 can be found in up to 50% of MPNST.1
A thoracic CT scan can frequently determine the preference for a PNST on the basis of the presence of a nerve entering and exiting the tumor in an anatomic region where this can be suspected.3 The uptake of intravenously administrated contrast demonstrates that the lesion is vascular, which is, in some cases, the only radiologic sign to separate solid tumors from mediastinal cysts. Although certain findings on thoracic CT, such as a smaller size, homogeneous uptake of intravenous contrast, and a well-distinguishable margin, might point in the direction of a benign tumor, radiologic characteristics can and should never be interpreted as being pathognomonical.3
Although not more decisive than CT in the discrimination between benign and MPNSTs, magnetic resonance can anatomically separate the mass from its adjacent structures, can evaluate its vascular supply, can judge whether there is locoregional extension, and can discriminate better between a mass and a cyst.4
The scientific strength for the use of fluoro-deoxy-glucose positron emission tomography in the diagnosis of PNST is limited. A meta-analysis concluded that fluoro-deoxy-glucose positron emission tomography has the potential to discriminate between sarcoma and benign tumors, but it remains unreliable to differentiate between benign tumors and low-grade sarcomas (such as MPNST).5 More recently, a maximum standardized uptake value (SUVmax) cutoff ≥6.1 separated MPNST from BPNST with a sensitivity of 94% and a specificity of 91%.6 By receiver operating characteristic curve analysis, the accuracy to predict the distinction between a benign and a MPNST was 97%.6 However, the difference between MPNST and schwannoma was less prominent than between MPNST and neurofibroma.
A pathologic study on the value of fine needle aspiration biopsy in mediastinal spindle cell tumors concluded that properly prepared smears and cell blocks are essential and ancillary immunohistochemical stains are valuable tools in evaluating these lesions, but a definite diagnosis based on cytologic features may not be rendered in all cases, given the wide differential diagnosis when spindle cells are encountered on fine needle aspiration.7
Performing EBUS-TBNA in these lesions could be seen as a minimally invasive technique to obtain a tissue diagnosis. In our case, EBUS-TBNA, enabled us to rule out more frequently seen causes of masses seen in the middle mediastinal compartment, such as lymphoma, thymoma, teratoma, and malignant lymph nodes, but our case demonstrates that cytology alone is not always enough to differentiate between benign and MPNST. The addition of a clinicoradiologic correlation is extremely important in suggesting the diagnostic possibilities and prompting further clinical action. Only the surgical resection specimen is fully able to differentiate between a cellular schwannoma and a MPNST, as demonstrated in our case.
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