CT imaging features of paravertebral mediastinal malignant peripheral nerve sheath tumour

42-year-old male patient presented with complaints of posterior chest pain and tingling on inspiration for 2 months. No fever, cough, haemoptysis, trauma. Mild intermittent shortness of breath was noted. On examination there was no obvious localised swelling, warmth or redness. Mild tenderness on deep palpation was noted. The patient is a chronic smoker. No other systemic symptoms. Laboratory studies were within normal limits. The physician advised CECT chest to evaluate further.

CT thorax shows well-defined soft tissue density lobulated solid mass lesion within the superior segment of the right upper lobe in the right paravertebral/subpleural location. It shows heterogeneous internal post-contrast enhancement with few tiny necrotic areas within (Fig. 1, 2). The lesion shows osseous infiltration into adjacent right posterior 5^th and 6^th ribs and 5^th intercostal space (Fig. 2, 4). On sagittal image, the lesion shows tapering end posteriorly within intercostal space (Fig. 3). No obvious bony erosions or destruction within adjacent vertebra or widening of neural foramina. No obvious posterior chest wall extension or paraspinal muscle infiltration of the lesion. An enlarged partially necrotic lymph node was noted in the precarinal region (Fig. 6). No other significant abnormality is noted within the rest of both lung parenchyma, mediastinal great vessels and axillary region. Unfortunately we were unable to perform an MRI scan of the thorax as the patient had declined.

Posterior mediastinal masses in paravertebral location can be seen arising from sympathetic ganglia, nerve roots, lymph nodes, parasympathetic chain, thoracic duct, descending thoracic aorta, small vessels and the vertebrae. Most masses in the posterior mediastinum (paravertebral location) are neurogenic in nature. These can arise from the sympathetic ganglia (e.g. neuroblastoma) or from the nerve roots (e.g. schwannoma or neurofibroma).
Peripheral nerve sheath tumour can be benign or malignant. It arises from nerve sheaths of peripheral nerves. It can be found anywhere predominantly in limbs, Paravertebral mediastinum, along with chest wall or posterior neck soft tissue.  The benign tumour is slow-growing and non-infiltrative whereas rapid growth or infiltration into adjacent structures or organs indicates malignant transformation [1]. Posterior mediastinal nerve sheath tumours are most common posterior mediastinal masses. Most of the tumours are benign. However, malignant transformation is rare. Clinically it presents as localised pain or radiating pain with or without visible posterior chest swelling and tenderness. Chest pain on phases of respiration or dyspnoea are also an atypical presentation. Systemic metastasis is very rare but most commonly spreads by haematogenous route to the lungs [3]. Most of the tumours present as part of the Neurofibromatosis -I (NF-I) spectrum [1].
Imaging features: Nerve sheath tumour appears as well-defined homogeneous or heterogeneous solid elongated or oval/rounded mass lesion within the posterior mediastinum in paravertebral location. Small tumours may appear as homogeneous isodense or hypodense mass lesion with tapering end whereas large tumors show heterogeneous density due to internal haemorrhage, necrosis or cystic degeneration on CT imaging [1, 2]. MRI imaging modality is the gold standard for diagnosis due to better soft tissue contrast. It can better detect the neural foraminal extension of the tumour. These tumours appear isointense in T1W and variable hyperintense in signal intensities on T2W and Fat saturated MRI images.  Heterogeneous central or target-like peripheral contrast enhancement are typical characteristics on CT and MRI imaging because of variable internal vascular myxoid, cellular and fibrous contents [3]. Split fat sign can be seen in the benign form (rim of displaced perilesional fat which surrounds a neurovascular bundle) as it is lost in malignant transformation [2, 1]. PET-CT study shows intense internal FDG uptake indicating malignant transformation. However, trace uptake can also be seen within benign tumours. Imaging studies can’t differentiate between benign and malignant nerve sheath tumours except for infiltration into adjacent structures. Diagnosis has to be confirmed by tissue biopsy and histopathological analysis [1]. CT and MRI imaging modalities can only detect size and extent of the tumours, the adjacent vertebral, rib or chest wall infiltration and are useful for for pre-operative planning.    
The treatment strategy includes primarily surgical excision with or without localised radiation therapy of neoadjuvant chemotherapy. Neoadjuvant chemotherapy is reserved mainly for inoperable lesions by reducing tumour size and extent preoperatively to make them operable. Postoperative localised radiation therapy is given to reduce local recurrence in case of partially resected aggressive tumours [1]. Multimodality treatment approach should be applied in case of infiltrating or metastatic tumours.
In our case, the tumour was biopsied under CT guidance and the patient was operated with almost complete resection of the tumour and post-operative radiotherapy application. An enlarged precarinal lymph node was also biopsied under guidance of bronchoscopy and turned out to be positive.

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