Posterior Mediastinal Mass

Neural Tumors

The neural tumors constitute the largest group of posterior mediastinal and posterior gutter masses ( Chart 12.1 ) ⁶¹ and are derived from nerve roots, intercostal nerves, or sympathetic ganglia. Nerve root tumors are schwannomas or neurofibromas. The sympathetic ganglion tumors include the neuroblastomas, ganglioneuroblastomas, and ganglioneuromas. Paragangliomas, including chemodectoma and pheochromocytoma, are infrequent causes of posterior mediastinal masses. ⁴⁵⁹

Schwannomas are derived from the sheath of Schwann, which forms the connective tissues of the nerve root. There are no nerve cells in a schwannoma. This is in contrast to the true neurofibroma, which contains both Schwann cells and nerve cells. Before 1955, distinction between these two tumors was not generally made. This suggested that neurofibromas are the most common cause of posterior mediastinal masses. The schwannomas are much more common than true neurofibromas and are the most common of all the neural tumors. Most schwannomas and neurofibromas are benign tumors. The very rare malignant nerve root tumors were previously classified as a malignant schwannoma, regardless of the presence of nerve cells in the tumor, but have been reclassified as malignant nerve sheath tumors.

Neuroblastomas are highly malignant, undifferentiated, small round cell tumors that originate from the sympathetic ganglia. In contrast, the ganglioneuromas, which are also derived from the sympathetic ganglia, are completely benign. ¹²³ Mature connective tissues, which resemble the connective tissues of schwannomas, are the primary tissues in ganglioneuromas. The presence of mature ganglion cells is the histologic feature that distinguishes schwannomas from ganglioneuromas. Ganglioneuroblastomas are mixed cell tumors that not only contain mature, well-differentiated ganglion cells, but also connective tissues and undifferentiated round cells. In other words, ganglioneuroblastomas have features of both ganglioneuromas, which are completely benign, and neuroblastomas, which are highly malignant. These tumors have a better prognosis than the neuroblastomas, but must be regarded as malignant tumors. It is true, however, that cases of spontaneous maturation have been observed. Cushing and Wolbach ¹⁰⁵ have described a case that matured from a neuroblastoma to a completely differentiated ganglioneuroma; unfortunately, this is a very rare event.

The radiologic presentation of most neural tumors is a homogeneous opaque mass in the posterior mediastinum. A small percentage of these tumors contain calcification. 127, 459 Calcification should be evenly distributed through a mass when the lesion is viewed in both the posteroanterior (PA) and lateral projections. The calcification often assumes a speckled pattern and may vary considerably in quantity ( Fig. 12.2, A and B ).

There is a striking difference between the shape of nerve root tumors and tumors of the ganglion series. 61, 459, 586 Of nerve root tumors, 80% appear as round masses (see Fig. 12.1, A-C ), whereas 80% of ganglion series tumors appear as vertically oriented, elongated masses ( Fig. 12.3 ). An additional feature that is helpful in distinguishing these masses is a tapered border, which suggests a ganglion series tumor. In contrast, round nerve root tumors tend to have a sulcus, which may indicate a more lateral position. This may suggest that the tumor is arising from the intercostal nerve. In Chapter 2 , schwannoma was mentioned as a possible cause of a chest wall mass. (Answer to question 1 is c .)

A variety of bone abnormalities may be observed with the neural tumors, including rib spreading, erosion, and destruction. Neural foramina enlargement indicates extension of the mass into the neural canal (see Fig. 12.1, A-C ). These tumors have been described as dumbbell lesions. ¹³⁸ Patients with neural foramina enlargement typically have neurologic deficits caused by compression of the spinal cord. ⁸ This complication usually results from benign nerve root tumors, but it may also occur when a large neuroblastoma extends posteriorly. ³⁴ Slow-growing benign tumors tend to erode bone, whereas malignant tumors invade and destroy bone. Erosion caused by a benign tumor may lead to a scalloped sclerotic defect in the posterior aspect of the vertebral body. Scoliosis is seen with very large masses, both benign and malignant, but rib spreading with erosion should suggest a malignant tumor ( Fig. 12.4 ).

Age is the most important clinical feature in the differential diagnosis of posterior mediastinal masses. In patients younger than 1 year, a posterior mediastinal mass is almost certainly a neuroblastoma. After 10 years of age, neuroblastomas are rare. Ganglioneuroblastomas typically occur in children between the ages of 1 and 10 years; ganglioneuromas typically occur in those in the 6- to 15-year-old age group. All the ganglion series tumors are relatively infrequent in adults, although ganglioneuromas are observed in people up to the age of 50 years. In contrast, schwannomas and neurofibromas are infrequent in childhood, occurring most frequently in the third and fourth decades. ⁴⁵⁹ (Answer to question 2 is c .)

Neurofibromatosis (von Recklinghausen disease) may also be associated with posterior mediastinal masses. ³⁸³ As many as 30% of patients with true neurofibromas may have neurofibromatosis. As mentioned earlier, the overall incidence of malignancy in the nerve root tumors is very low, but there is a trend to malignant transformation in patients with von Recklinghausen disease. Schwannomas and ganglioneuromas have also been reported in this group of patients, but the association is less frequent. Ganglioneuromas more commonly occur as isolated, posterior mediastinal masses.

Lymphadenopathy

Lymphadenopathy should not be overlooked as a cause of a posterior mediastinal mass. Lung cancer may metastasize to the posterior lymph nodes and should be considered in the presence of other signs of a primary tumor, such as atelectasis or hilar or paratracheal masses. The mass may be quite large and involve both the middle and posterior compartments. Lymphoma is an uncommon but important cause of posterior mediastinal adenopathy and must be considered in patients with systemic symptoms, including weight loss and low-grade fever. The additional finding of associated hilar adenopathy in these patients could be strongly suggestive of lymphoma. Conversely, the combination of a paraspinal mass and bilateral, symmetric hilar adenopathy in an asymptomatic patient raises the possibility of sarcoidosis. Paraspinal masses in patients with sarcoidosis have been reported, but are rare. Often, the differentiation of sarcoidosis from lymphoma can be made by considering the clinical presentation of the patient. In the patient with lymphoma, it is unlikely for her or him to have extensive hilar and mediastinal adenopathy and remain asymptomatic, whereas the patient with sarcoidosis may typically be asymptomatic.