True mediastinal mass difficult to distinguish from technical factors (AP, supine, rotation), but is aided by mass effect, increased density, obsturation of contours (knob and paratracheal stripe).
Presense of fat indicates hernia, lipoma, teratoma, epicardial fat pad or thymolipoma. Fat-fluid level diagnositc of mature teratoma.
Superior mediastinal/thoracic inlet masses:
- Thyroid – 80% anterior to trachea, 20% right or posterolateral to trachea. Coarse, clumped calcification. On CT well-defined, continuity with cervical thyroid, cystic/necrotic areas, baseline high HU (intrinsic iodine), intense enhancement (>25HU) due to hypervascularity and prolonged enhancement (active uptake of iodine). Goiter, malignancy, thyromegaly from thyroiditis.
- Parathyroid – 2% ectopic in or adjacent to thymus, most lesions <30mm are adenomas, rarely hyperplastic or parathyroid carcinoma.
- Lymphangioma – Uncommon tumour of dilated lymphatics. Cystic/cavernous form (cystic hygroma) usually infants, associated with Turners, trisomy 13/18/21. Extend from neck -> anterior mediastinum. Occasionally in older patients primary in anterior mediastinum. Benign, insinuate between structures causing resection difficult, recur. Well-defined cystic mass.
Anterior mediastinum (4 T’s):
- (Terrible) Lymphoma – Hodgkin, non-Hodgkin
- Thymus (>40yo) – Thymoma, cyst, thymolipoma, hyperplasia, neuroendocrine tumour, carcinoma, lymphoma
- (‘Teratoma’) Germ cell tumour (<40yo) – Teratoma, seminoma, emryonal cell carcinoma, endodermal sinus tumour, choriocarcinoma
- Thyroid and ectopic parathyroid masses
- Mesenchymal tumours – Lipoma, haemangioma, leiomyoma, liposarcoma, angiosarcoma
- Lymph node masses – Bronchogenic carcinoma, extrathoracic malignancy, lymphoma, sarcoidosis, mycobacterial, fungal, angiofollicular LN hyperplasia (Castleman disease), angioimmunoblastic lymphadenopathy. >10mm short axis diameter. See [[Chest#mediastinal lymph nodes|ATS Nodal Stations]]. Central calcification – mycobacteria, fungal; peripheral/eggshell calcification – Silicosis, sarcoid. Hypervascular – carcinoid/small cell, Kaposi sarcoma, RCC/thyroid mets, Castleman disease. Necrosis – mycobacteria, fungal, SCC/seminoma/lymphoma metastases. Leukaemia (esp T-lymphocyte) usually confined to mid mediastinal and hilar nodes.
- Foregut and mesothelial cysts – Bronchogenic cyst, pericardial cyst
- Tracheal and central bronchial tumours
- Diaphragmatic hernias
- Vascular lesions – Double/right arch, tortuous innominate/subclavian, aortic aneurysm, dilated azygos/hemiazygos/SVC, left SVC, dilated left superior intercostal v, dilated MPA
- Neurogenic tumours – Schwannoma, neurofibroma, ganglioneuroma, ganglioneuroblastoma, neuroblastoma, paraganglioma
- Oesophageal lesions – Cyst, diverticulum, neoplasm, dilatation, varices, hernia
- Vertebral lesions – Paraspinal haematoma/abscess, mets/myeloma/lymphoma, osteophytes
- Extramedullary haematopoiesis – Inefective production or excessive destruction of erythrocytes. Expansion of medullary spaces and cysts in long bones and vetebrae with lobulated paraspinal hyperplastic bone marrow tissue esp lower T/upper L spine.
- Lateral thoracic meningocoele – Herniation of meninges through intervertebral foramen, asymptomatic. Associated with NF1. Round well-defined mass indistinguishable from neurofibroma; may be associated with rib erosion, enlarged neural foramen, kyphoscloliosis (meningocoele usually at apex of convexity). MR or XR/CT melography.
- Pancreatic pseudocyst – Extension via aortic or oesophageal hiatus.
- Rarely malignant lymph node.
Thymus is derived from the 3rd ± 4th pharyngeal pouch. Grows until puberty then progressive involution. Fibrous capsule and septae divide lobes into lobules with outer cortex and central medulla. Mostly thymic epithelial cells and immature T lymphocytes. Abnormalities include:
- Thymic hypoplasia/aplasia – DiGeorge syndrome
- Thymic (follicular) hyperplasia – Appearance of B-cell germinal centres in the thymus. The gland is usually normal or diffusely enlarged, occasionally a mass (indistinguishable from thymoma). In chronic inflammatory/immunological states esp myasthenia gravis, Grave’s, SLE, scleroderma, RA. May be a rebound effect in children in response to stress or stopping chemotherapy.
- Thymic cyst – Congenital (rare; unilocular, remnant of thymopharyngeal duct) or acquired (postinflammatory from AIDS, radiation/surgery, autoimmune). May have calcified rim. Difficult to distinguish from thymoma (esp if multilocular), cystic degeneration of thymoma/lymphoma, lymphangioma, germ cell tumour.
- Thymic epithelial neoplasm – >40yo, rare <20yo, M=F, associated with myasthenia gravis (10-28% have thymoma, 30-54% thymomas are from MG) and other autoimune disseases. Intermixed lymphocytes. Classified into hitological A (thymoma), AB, B1, B2, B3 or C (thymic carcinoma, poor prognosis).
- Noninvasive thymoma – Round/oval, smooth/lobulated, typically homogeneous near base of heart (anterior to aorta, superior to RVOT). Displaces rather than follows contours (cf normal thymus). There is no such thng as a benign thymoma.
- Invasive thymoma – Cytologically benign but locally invasive. 20-25% of all thymomas. Penetrate through the capsule into surrounding structures, invasion often difficult to find on imaging or histology. After resection may have late recurrence (local or metastatic) years later.
- Thymic carcinoma – Most are SCC, others lymphoepithelioma-like carcinoma. Irregular margins, heterogeneous enhancement, regions of necrosis, LNs, coarse calcification. Local invasion to pleura, lung, pericardium, chest wall, diaphragm, great vessels (10-15%). Drop metastases to dependent pleural space. Extrathoracic metastases rare, but can invade the retroperitoneum.
- Thymolipoma – Rare, benign, primarily fat with rests of normal tissue, asymptomatic, large at detection, pliable (follows contours). Resection curable.
- Thymic carcinoid – Rare malignant neuroendocrine tumour from thymic cells of neural crest origin. 40% secrete ACTH causing Cushing syndrome. Carcinoid syndrome uncommon. Indistinguishable from thymoma on CT.
- Thymic lymphoma – Suggested if there is lymphadenopathy. Thymus involved in 40-50% of nodular sclerosing Hodgkin disease.
See Haematopoietic System Lymphoma
(Angiofollicular lymph node hyperplasia). Noncancerous B cell growth in LN, usually middle and posterior mediastinum. Hyaline vascular type localised to one region, multiple germinal centres and small vessles with hyalinized walls with ‘lollipop’ appearance on histology.Hypervascular, may have central stellate nonenhancement. Occasionally calcifies. Cured by resection.
Rare, older patients. Constitusional Sx, LN, hepatosplenomegaly, rash, haemolytic anaemia, hypergammaglobulinaemia. Chronic inflammatory infiltrate. Hypervascular. Causes immunodeficiency, 1/3 develop lymphoma. AILD (anti-immunoblastic lymphadenopathy with dysproteinaemia) = angioimmunoblastic lymphoma.
Germ Cell Tumours
20-40yo, M>F. Teratoma (60-70%), seminoma, choriocarcinoma, endodermal sinus tumour, or embryonal cell carcinoma. Germ cells arrested in anterior mediastinum en route to gonads, histologically indistinguishable gonadal tumours (hence diagnosis of these being primary requires normal gonads, no retroperitoneal LN). 10% posterior mediastinum. Benign lesions are round/oval, smooth contour. Malignant lesions are irregular, lobulated or ill-defined margins. Calcification in 33-55%. GCTs in males are highly likely to be metastatic from the testes, so scrotal US required.
- Teratomas cystic/mature or solid (usually malignant) – Contains ectodermal, mesodermal and endodermal elements; may contain bone, teeth, fat, rarely fat-fluid level.
- Seminoma, choriocarcinoma, endodermal sinus (yolk sac) tumours – Malignant, usually young men. Large lobulated mass with possible haemorrhage, calcification or necrosis. Elevated alpha-fetoprotein or beta-HCG, gynaecomastia.
- Lipomas – Most common anteriorly. Well defined fatty HU<–50 mass.
- Haemangiomas – Smooth/lobulated, phleboliths. Associated with hereditary haemorrhagic telengiectasia.
- Angiosarcomas – Indistiguishable from other invasive tumours.
- Leiomyomas – Rare benign from smoooth muscle.
- Mesenchymoma – More than one mesenchymal element.
From alveoli (Valsalva maneuver, positive pressure ventilation esp with ARDS stiff lungs, marijuana/NO/cocaine, asthma), tracheobronchial tree (bronchial stump dehiscence, laceration, perforated oesophagus, fistula (tracheal/oesophageal malignancy, infection), recent surgery, pneumoperitoneum/retroperitoneum, subcutaneous emphysema, stab wound, laryngeal fracture. Macklin effect – increased intrathoracic/alveolar pressure causes rupture with gas -> bronchovascular interstitium -> hilum -> mediastinum; or less commonly peripherally ?through viscral pleura -> pneumothorax. Pneumomediastinum without infection usually does not require treatment; rarely causes tension (similar signs of tamponade). Curvilinear lucency lining structures esp left heart border with thin outer pleural line (cf ‘optic illusion’ Mach effect/bands from retinal reinforcement response, disappears when interface covered). Easier on lateral, lining aortic root or MPAs. Continuous diaphragm sign – between pericardium and diphragm, also seen in pneumopericardium.
- Oesophageal cancer usually advanced (absent serosa), squamous cell carcinoma. Abnormal azygo-oesophageal interface, widened mediastinum (tumour with proximal dilatation), thickened tracheo-oesophageal stripe, tracheal deviation/compression.
- Benign leiomyoma, fibroma and lipoma are smooth solitary, usually lower third forming obtuse margins with walls and don’t tend to obstruct.
- Pulsion/false diverticula are outpouchings through defects in muscle, at cervicothoracic junction (Zenker -> retro-oesophageal mass) or distal oesophagus. Air-fluid levels.
- Dilated oesophagus (achalasia, scleroderma, stricture, carcinoma) projects towards right, air-fluid level or thin line if air-filled.
- Varices are round/lobulated.
- Hiatus hernia may rarely contain omental fat, ascites or pancreatic pseudocyst.
- Congenital bronchogenic cysts – Young, assymptomatic. Anomalous budding of tracheobronchial tree, lined by respiratory epithelium; occasional cartilage and smooth muscle. Most close to carina. Round cystic hilar mass with imperceptible walls. Foregut cyst refers to those unable to be specifically characterised.
- Pericardial cysts from parietal layer, lined by mesothelial cells. Usually R anterior cardiophrenic angle. Pliable.
- Enteric cysts lined by enteric epithelium, eg oesophageal duplication cysts.
- Neuroenteric cysts communicate with spinal canal via canal of Kovalevski; associated with vertebral anomoly.
- Nerve sheath tumours (neurofibroma and schwannoma) – Usually adults, spindle cells from Schwann cell, intercostal nerves. If multiple, virtually diagnostic of neurofibromatosis. Neurofibroma is encapsulated, interspersed neurons. Schwannoma not encapsulated, no neuronal elements; 10% malignant. Smooth/lobulated mass, may erode adjacent vetebrae/rib, acute angles with posterior mediastinum/vertebrae. Dumbell neurofibroma – extension from paravertebral space into spinal canal via enlarged intervertebral foramen. High T2.
- Sympathetic ganglion tumours – Benign ganglioneuroma (young adults), ganglioneuroblastoma, malignant neuroblastoma (<5yo). Elongated, vertical orientated mass, bony erosion, 25% calcify. Raised catecholamines (urine metanephrine, vanillylmandelic acid).
- Paragangiomas – From aorticopulmonary (mid mediastinum) or aorticosympathetic (post) paraganglia. Chemodectomas (nonfunctioning) almost always AP window. Other paragangliomas (functioning) posterior sympathetic chain or heart/pericardium. Hypervascular, MIBG uptake.
Usually seriously unwell with CP, high fever, dyspnoea, sepsis. Bacterial infection, usually from oesophageal perforation (instrumentation, trauma, carcinoma, foreign body, corrosive ingestion, vomiting), cardiothracic surgery, extension from infection in neck/retropharyngeal space/lungs/pleural space/pericardium/spine. Boerhaave syndrome (prolonged vomiting) vertical tear distal left posterolateral oesophagus -> left hydropneumothorax and pneumoperitoneum. Widening of superior mediastinum, pleural effusion, pneumomediastinum, air-fluid levels. Ludwig angina – substernal chest pain by intramediastinal extension of deep neck infection. High mortality, requiring early diagnosis. Cx focal abscess formation, venous thrombosis, pneumothorax, pleural effusion/empyema, subphrenic abscess, osteomyelitis.
Chronic Sclerosing/Fibrosing Mediastinitis
Due to granulomatous infection Histoplasma capsulatum (?hypersensitivity to fungal antigen than leaks from infected LNs), TB, radiation, drugs, idiopathic (autoimmune, related to fibrosis in retroperitoneum, intraorbital fat and thyroid). Cx SVC obstruction (75% of symptomatic patients), obstruction of central pulmonary veins (pulmonary oedema, potentially fatal), tracheobronchial tree, oesophagus, PA HTN/cor pulmonale from narrowed PAs. Asymmetric lobulated widening of upper mediastinum (usually right), calcified LN (granulomatous infection), soft tissue density replacing normal mediastinal fat obliterating normal interfaces.
Trauma, central line placement, coagulopathy, aortic aneurysm/dissection, chronic haemodialysis, radiation vasculitis, bleeding into mediastinal mass. Cx SVC obstruction, extension into retropharyngeal space (neck stiffness, odynophagia, stridor), extension into pleural or extrapleural space, lungs via bronchovascular interstitium (mimics oedema). Focal or diffuse widening and increased density of mediastinum obscurring normal contours, flat/convex contour.
Benign, asymptomatic excessive fat in mediastinum, also paraspinal, chest wall, cardiophrenic angles/epicardial fat pads. Risks include obesity, Cushings, corticosteroids, unidentified cause in 50%. Smooth symmetric widening of superior mediastinum ± lobulated margins; structures not dislapced/narrowed. Heterogeneity in the fat suggests neoplastic/infection/haemorrhage/fibrotic inflitration. Multiple symmetric lipomatosis (rare) spares cardiophrenic angles, paraspinal area and anterior mediastinum, associated with periscapular lipomas and often compresses/displaces trachea.
Hilar masses cause enlargement, increased density, lobulation, distortion/narrowing of central bronchi, blunting of the sharp right hilar angle (sup pulm v. and interlobar a.). Hilum overlay sign – normal vessels seen through dense hilum, indicating mass superimposed rather than in hilum. Hilum convergence sign – loss of vascular sillhouette.
- Unilateral hilar enlargement – Malignancy, infection (TB, histoplasmosis, coccidioidomycosis, infectious mononucleosis, measles pneumonia), PA enlargement (poststenotic dilatation, aneurysms, thrombus or tumour), bronchogenic cyst.
- Bilateral hilar enlargement – Above conditions and sarcoidosis (right paratracheal -> right hilar -> left hilar; punctate or eggshell calcification), berylliosis (eggshell calcification), silicosis, CF.
- Small hila – Atelectasis, lobectomy, severe hyperinflation, reduced pulmonary blood flow (congenital pulmonary outflow obstruction), hypoplasia of the PA, invasion by tumour, obstruction from fibrosing mediastinitis.
- Hilar nodes enlarged if >10mm maximum short axis diameter. Tumour or inflammatory process may extend through LN capsule to other contiguous nodes, difficult to distinguish from primary hilar carcinoma. LN metastases in bronchogenic carcinoma, lymphoma (bilateral), melanoma, GU, (renal and testicular), H&N (skin, larynx, thyroid), breast.