By Dr Aniket Agrawal
We are back with a new topic discussion for Radiology Residents with Dr Aniket Agrawal for our FOAM(Free online access medicine) Intiative. Dr Aniket is a First year resident in DNB Radiology at Diwanchand Imaging centre, Delhi . He will crisply describe about Anterior Mediastinal masses spaces . You can discuss the topic in the comment section below this blog and also at Freeassociation.in Forums Here
Boundaries
• Anteriorly – sternum
• Posteriorly – Brachiocephalic vessels, pericardium, and ascending aorta
• Superior – thoracic inlet
• Inferior – diaphragm
So, heart and trachea lie in the middle mediastinum.
Classification
Categorization of Anterior Mediastinal Masses according to Predominant CT Attenuation Values:
• FAT ATTENUATION
• WATER ATTENUATION
• SOFT TISSUE ATTENUATION
• CALCIFIC ATTENUATION
• VASCULAR
Classification according to CT attenuation values helps in a better understanding of the masses.
FAT ATTENUATION
• LIPOMA
• LIPOSARCOMA
• MEDIASTINAL LIPOMATOSIS
• THYMOLIPOMA
• OMENTAL HERNIA
• TERATOMA
Lipoma
• Benign
• Encapsulated
CT : characteristic homogeneous fat attenuation (usually −50 to −100 HU)
• no contrast enhancement
• well-defined margins
MRI may confirm the fatty nature of the tumor, which produces high T1, intermediate on T2 and low T2 signal intensity on fat-suppressed images.
Liposarcoma
• Mesenchymal malignant tumor with fatty differentiation.
• On CT, the appearance ranges from a fatty lesion to a pure soft-tissue tumor.
• In general- appears as inhomogeneously enhancing mass with variable amounts of fat and soft-tissue density.
• MRI is helpful to detect the fatty component and contrast enhancement.
Mediastinal Lipomatosis
• Excessive deposition of unencapsulated fat in the mediastinum.
• Exogenous steroid intake and obesity.
• Cushing syndrome.
• Adrenocorticotropic hormone–producing tumors.
• Xray: Smooth widening of the mediastinum, which may be accompanied by prominent epicardial fat pads.
• CT shows diffuse homogeneous fat-attenuation.
• The presence of a soft-tissue component, septa, capsule, and contrast enhancement should suggest an underlying tumor.
Omental Hernia
• Congenital Morgagni hernia
• Acquired postoperative or post-traumatic diaphragmatic defects
Thymolipoma
• Represents 10% of thymic neoplasms.
• Benign
• Composed mainly of fat and variable amounts of thymic tissue.
CT:
• Fat constitutes 50-85% of this mass.
• Encapsulated
• Non-enhancing linear densities, which represent thymic tissue, are interwoven within the fat.
MRI:
• Normal thymus has same intensity as that of skeletal muscle.
• A thymolipoma is of high signal intensity on both T1 and T2 and of low intensity on fat supressed images.
• Visualization of a large predominantly solid necrotic, hemorrhagic and enhancing lesion with elevated α-fetoprotein should raise concern for malignant teratoma.
• MRI – high signal intensity on T1- and T2-weighted sequences and low signal intensity on fat-suppressed imaging.
Note: other germ cell tumors produce predominantly soft tissue attenuation.
• Fat is hyperintense on T1 and intermediate to hyperintense on T2.
WATER ATTENUATION
• THYMIC CYST
• LYMPHANGIOMA
• ABSCESS
• OTHERS- pleuropericardial cyst, foregut duplication cyst, cystic teratoma, cystic degeneration of malignancy, pancreatic pseudocyst- these are very rare in anterior mediastinum.
• THYMIC CYSTS
o Congenital thymic cysts – simple, unilocular, and thin-walled lesions containing clear fluid without pathologic evidence of inflammation.
o Acquired thymic cysts –
• Are multilocular with variable wall thickness with pathologic evidence of inflammation and tend to recur after resection and may be associated with thymic malignancy.
• associated with myasthenia gravis, SLE, Sjögren syndrome, aplastic anemia, radiation therapy, Hodgkin disease, AIDS in children.
Complicated thymic cyst
• If complicated by hemorrhage or infection, acquired thymic cyst may resemble a solid mass and show higher CT attenuation and even calcification within its wall.
• MRI- high intensity on both T1 and T2.
Lymphangioma
• <1% cases occur in thorax and involve anterior mediastinum most commonly.
• Chest xray: well defined mass with chylous pleural effusion.
CT:
• homogeneous lobulated lesion
• Calcification and contrast enhancement are uncommon.
MRI:
• high signal on T2 with internal septations.
• variable on T1 depending upon amount of proteinaceous and hemorrhagic contents of the lesion.
Abscess
• When the collection contains air and has a thick rim-enhancing wall.
SOFT TISSUE ATTENUATION
• Thymic hyperplasia or neoplasm
• Thyroid goiter
• Ectopic parathyroid adenoma
• Germ cell tumor
• Mediastinitis
• Acute Fibrosing Lymphadenopathy
• Lymphoma
• Metastasis
• Others: hernia, Castleman disease, sarcoma
Thymic disorders
• Normal thymus should not be visible during adult life.
• Normal shapes: Bilobed, inverted v shape, quadrilateral
• Normal borders – straight/ concave
• Maximum thickness should be <13mm.
• As thymus involutes over time, it is replaced by fat.
• On MRI, the normal thymus has a signal intensity similar to muscle on both T1- and T2-weighted images.
• Due to atrophy, the signal intensity becomes higher on both sequences due to fatty infiltration. In- and out-of-phase sequence- detects fat content.
• Normal thymus gland: most common cause of mediastinal abnormality in infants, which projects to one side, often the right side.
Causes of decrease in thymic size:
• severe neonatal infection
• steroid therapy
• major surgery
• but may reappear after recovery from illness.
• Completely absent in DiGeorge syndrome.
Causes of enlargement of thymus:
• Thymoma – most common, 30% are invasive
• Hyperplasia
• Thymic carcinoma
• Lymphoma
• Carcinoid
• Germ cell tumor
Facts
• ~10-25% of patients with myasthenia have thymoma.
• >50% of patients with myasthenia have thymic hyperplasia.
• ~25-50% of patients with a thymoma have myasthenia.
• ~25-50% of patients with a thymoma have red cell aplasia.
• ~10% of patients with a thymoma have hypogammaglobulinemia.
Staging of thymoma on CT/MRI
• Stage 1: no capsular invasion
• Stage 2: capsular invasion
• Stage 3: invasion of mediastinal structures or lung
• Stage 4: (a) dissemination within thorax
(b) distant metastases
Thymic hyperplasia
• True hyperplasia and lymphofollicular hyperplasia.
• True thymic hyperplasia is an increase in size and weight of the gland with normal histology. This condition is commonly due to a rebound from recent stress, such as pneumonia, corticosteroid therapy, radiation therapy, chemotherapy, surgery, or burns.
• Lymphofollicular hyperplasia refers to the presence of a hyperplastic lymphoid germinal center in the medulla of the gland.
Also seen in:
• >50% of patients with myasthenia gravis
• Thyrotoxicosis
• Systemic lupus erythematosus
• Polyarteritis nodosa
• Addison disease
• On CT, thymic hyperplasia usually appears as symmetric enlargement of the gland.
• Asymmetric enlargement raises the possibility of a thymoma.
• Thus, we see that in thymic hyperplasia, the thymus is normal in shape but enlarged in size.
Malignancy??
• Convex round thymus
• Irregular margin
• Heterogenous contents- necrosis, hemorrhage, calcification, cystic changes
• Note : if MRI detects fat within thymus, it suggests thymic hyperplasia.
Thyroid goiter
• On xray- homogeneous smooth sharply marginated mass in a retrosternal, paratracheal or retrotracheal location.
• NCCT: hyperattenuating mass, focal calcifications.
• CECT: early, prolonged enhancement.
Mediastinitis
Acute mediastinitis
• Emergency condition d/t esophageal rupture or any surgical complication esp sternotomy.
• Presents with fever, tachycardia, chills, sepsis.
• Retrosternal pain
• Subcutaneous emphysema
• Chest xray: mediastinal widening, diffuse/focal gas bubbles, pleural effusion, mediastinal air-fluid levels or a soft tissue mass.
CT- modality of choice
• Fat stranding
• Lymphadenopathy
• Abscess and empyema.
• Defines the extension of the process
• Determines the underlying cause
• A diagnosis of esophageal perforation should be suggested if there is esophageal wall thickening and the presence of a fluid collection, extraluminal air, extravasation of contrast material into the pleural space or mediastinum, pneumomediastinum, or pneumothorax.
• Mediastinitis developing acutely after sternotomy can cause sternal dehiscence and misaligned sternum wires.
• CT also is the modality of choice for showing direct extension of infection from osteomyelitis and infections of the head and neck.
Chronic mediastinitis
• Also known as fibrosing mediastinitis.
• Most often affects middle mediastinum.
Causes:
• Infections: TB, histoplasmosis
• Autoimmune diseases
• Lymphoma
• Radiotherapy
• Drugs: methylsergide
CT:
• Soft tissue mass that infiltrates the mediastinum and obliterates adjacent structures.
• Lymphadenopathy
• Calcification
• Mediastinal widening
Parathyroid adenoma
• Initial Investigation: USG and Tc-99m sestamibi scan.
• If inconclusive, then CT/ MRI.
• CT: 1-3cm oval/ round well defined mass of soft tissue attenuation, which enhances on contrast.
• May be difficult to differentiate from a lymph node.
• MRI: T1- iso to hypointense
T2- iso to hyperintense,
as compared to thyroid and enhances avidly.
Germ cell tumors
Seminomatous:
• Large
• Homogeneous
• non-calcified
Non seminomatous:
• Heterogeneous
• Ill-defined margins
• May contain areas of necrosis and hemorrhage
Lymphadenopathy
• In Lymphoma- L.N. coalesce into large mass
• HL: homogeneous soft tissue mass with moderate contrast enhancement.
• May contain cystic changes and necrosis.
• NHL: more vascular involvement and lymphadenopathy.
CALCIFIC ATTENUATION
1) Malignancy
• Teratoma
• Metastasis from serous or mucinous tumors
• Metastasis from bone tumor
• Treated lymphoma
• Thymoma
2) Aneurysm of ascending aorta
3) Goiter: in 75% cases
4) Granulomatous infection
• TB
• Histoplasmosis
• Sarcoidosis(egg shell calcification)
• Silicosis(egg shell calcification)
VASCULAR STRUCTURES
• Aneurysm of ascending aorta
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