A 46-year-old woman with a past medical history of sickle cell trait and thalassemia intermedia (Hb S-thal) was referred to a local hematologist to establish care after moving to the area. The patient had never required a transfusion and her hemoglobin had been stable in the 7-8 g/dl range. The patient reported a history of an abnormal chest radiograph as well as an episode of slurred speech for which she had undergone a workup to exclude an acute stroke. The patient’s past medical history, review of systems, and physical examination were otherwise unremarkable. The patient was referred for a follow-up PA chest radiograph (Figure 1) and a subsequent chest CT (Figure 2). The patient’s MRI of the brain was also obtained (Figure 3).
Bilateral posterior mediastinal masses
Expansion of the diploic space
Nervous System
Neurogenic tumors
Infectious spondylodiscitis
Meningocele
Neuroenteric cyst
Vascular System
Descending thoracic aortic aneurysm and/or dissection
Esophageal varices
Dilated azygous or hemiazygous veins
Gastrointestinal System
Esophageal neoplasms
Esophageal duplication cysts
Dilated esophagus
Hematologic/Lymphatic Systems
Lymphadenopathy
Lymphoma
Extramedullary hematopoiesis
Neoplasm
Metastases
Soft-tissue tumors
The posterior mediastinum is anatomically defined superiorly by the thoracic inlet, inferiorly by the diaphragm, anteriorly by the pericardium and great vessels, laterally by the right and left parietal pleura, and posteriorly by the prevertebral fascia and the paravertebral space.1,2
A wide range of anatomic structures reside in the posterior mediastinum and contribute to the initial consideration of a broad differential diagnosis including pathology of the spine, vasculature, esophagus, lymph nodes, and soft tissues. Additionally, pathologic processes involving extrathoracic structures such as a thyroid goiter or a pancreatic pseudocyst may also extend into the posterior mediastinum.3 However, the CT appearance of our case significantly narrows these initial diagnostic considerations to a focused differential of spine/nerve pathology, vascular abnormalities, and a hematologic/lymphatic process.
Neurogenic tumors are the most common type of posterior mediastinal mass.2 They can be broken into three distinct categories based on location and anatomic origin including peripheral nerve sheath tumors, sympathetic ganglia tumors, and paragangliomas.2,4,5
Peripheral nerve sheath tumors present as round or ovoid masses that silhouette the paraspinal lines on radiographs and typically span one or two vertebral levels. Most of these lesions exhibit homogeneous soft-tissue attenuation at CT. They can, however, exhibit variable attenuation as they may sometimes contain areas of necrosis, calcification, hemorrhage, fat, or areas of cystic degeneration. These lesions also exhibit variable patterns of enhancement on postcontrast imaging.
Extrinsic erosion, scalloping of adjacent ribs or vertebral bodies, as well as widening of adjacent neural foramen leading to a dumbbell-shaped soft-tissue mass are common findings associated with peripheral nerve sheath tumors. Schwannomas and neurofibromas are the most common types of neurogenic tumors in adults.
Sympathetic ganglia tumors most often consist of paraspinal masses that span multiple vertebral levels and often contain calcification. There are three forms of sympathetic ganglia tumors: neuroblastoma, ganglioneuroblastoma, and ganglioneuroma. Paragangliomas are neuroendocrine tumors that may occur anywhere from the base of the skull to the pelvis and are histologically similar to pheochromocytomas.6
MRI can be useful in differentiating various types of neurogenic tumors. Paragangliomas are comprised of intratumoral vessels that result in flow voids causing a classic “salt and pepper” appearance on T1-weighted images. Neurofibromas are characterized by a classic “target-like” appearance consisting of central low-signal intensity and peripheral high-signal intensity on T2-weighted imaging. Ganglioneuromas typically exhibit a whorled appearance on T2-weighted imaging.
Discitis-osteomyelitis typically consists of involvement of two contiguous vertebral bodies and the intervening disc. Increased T2/STIR signal is demonstrated in the marrow due to edema with corresponding decreased T1 signal intensity due to replacement of marrow fat by edema and enhancement on postgadolinium sequences.
Staphylococcus aureus is the most common causative bacteria. Other pathogens may include Strep viridans, gram-negative organisms such as salmonella, Enterobacter and E. coli, various fungi, and Mycobacterium tuberculosis.
Several features can help distinguish tuberculous spondylitis—known as Pott’s disease—from pyogenic spondylitis. Tuberculous infection is more likely to involve greater than two vertebral levels. It will often spare the intervertebral disc or only show mild disc involvement. There is often more extensive soft-tissue involvement and epidural abscess formation in the setting of tuberculous spondylitis. Osseous destruction leads to a sharply angulated thoracolumbar kyphosis known as a gibbus deformity.
Radiographs are often insensitive and typically only begin to show findings of disc space narrowing and irregularity after 2 to 4 weeks of infection. CT will demonstrate similar findings along with potential findings in the paraspinal soft tissues. CT with contrast may demonstrate enhancement of the disc as well as an epidural abscess. Bone scintigraphy lacks specificity. Leukocyte scintigraphy for vertebral osteomyelitis is limited, and Ga-67 scintigraphy is often preferred.7 F-18 fluorodeoxyglucose (FDG) PET/CT has a potential role in osteomyelitis as activated neutrophils and macrophages are hypermetabolic. Overall, however, MRI has the highest sensitivity and specificity of all modalities in diagnosing discitis-osteomyelitis.
Meningoceles result from protrusions of the meninges through a defect in the skull or spine. They can be characterized as congenital, iatrogenic (ie, following craniotomy or laminectomy), or spontaneous.
Neurenteric cysts are a rare form of foregut duplication cysts that result from incomplete resorption of the neurenteric canal. They most commonly occur in the thoracic region where they present as well-defined, low-attenuation lesions. They may exhibit variable signal intensity on both T1- and T2-weighted MR imaging and typically demonstrate peripheral rim enhancement on post- contrast imaging.
Aneurysms involving the descending thoracic aorta often present as an incidental posterior mediastinal mass on imaging. They are more common in men in their 50s and 60s. The descending thoracic aorta is considered aneurysmal when it reaches > 3 cm in diameter.8
Esophageal varices are common findings in patients with portal hypertension. In rare circumstances, esophageal varices can present as a posterior mediastinal mass when significantly engorged.
A myriad of congenital, physiologic, and pathologic etiologies can lead to dilatation of the azygous and hemiazygous veins. When these structures are dilated, they can cause widening of the right paratracheal and paraspinal soft tissues, thus mimicking a posterior mediastinal mass.
Lymphadenopathy due to metastatic disease or lymphoma may also result in a posterior mediastinal mass. However, extensive adenopathy isolated to the posterior mediastinum would be an unusual presentation for these processes.
Hematopoiesis normally occurs within the red bone marrow of the vertebrae, ribs, and long bones in adults. Failure can occur in the setting of myeloproliferative disorders such as chronic myelogenous leukemia, polycythemia vera, essential thrombocytosis, myelofibrosis, Hodgkin disease and hemoglobinopathies such as sickle cell disease, thalassemia, or hereditary spherocytosis.9 In these settings, former primary sites of erythropoiesis are reactivated.10 This process, termed extramedullary hematopoiesis, is more common in hemolytic anemias and sickle cell variants such as Hb S-thal, as opposed to the homozygous (Hb SS) or heterozygous (Hb SC) forms of sickle cell anemia.7 Extramedullary hematopoiesis most commonly results in enlargement of the spleen and liver. It can also involve the lymph nodes, adrenal glands, or skin. Less commonly, these patients may develop unilateral or bilateral, well-marginated, round or ovoid soft-tissue masses in the paraspinal regions of the thorax. Rib expansion along with a heterogeneous lacy appearance of the ribs and vertebral bodies often accompany these findings. Similar to active intramedullary hematopeietic tissue, sites of extramedullary hematopoiesis in the paraspinal regions of the thorax exhibit intermediate signal intensity on both T1 and T2-weighted sequences.7 Sites of extramedullary hematopoiesis can be mildly avid on F-18 FDG PET/CT.11 Marrow imaging with Tc-99m sulfur colloid can also be useful in confirming the hematopoietic nature of the paraspinal masses.7
Extramedullary hematopoiesis
In this case, bilateral, well-marginated, posterior soft-tissue mediastinal masses were demonstrated on a frontal chest radiograph. Associated osseous and extraosseous findings on the follow-up CT scan of the chest, as well as findings on prior imaging of the brain helped to arrive at the diagnosis of extramedullary hematopoiesis in this patient with a known history of chronic anemia due to sickle cell trait and thalassemia intermedia (Hb S-thal). Based on the combination of the patient’s past medical history and classic imaging findings, the need for further invasive workup with a percutaneous CT-guided biopsy of one of the paraspinal masses was deemed clinically unnecessary. However, in cases where diagnostic doubt remains, sulfur colloid scintigraphy and/or tissue sampling may be useful.
Gazaille R, Singh A, Nielson J. Bilateral Posterior Mediastinal Masses. J Am Osteopath Coll Radiol. 2020;9(4):25-28.
