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Radiology Essentials 104 : Anterior Mediastinal Masses

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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