Mediastinal Masses Radiology

The mediastinum is a high-yield "anatomy meets imaging" topic where the compartment in which a mass sits, the age of the patient, and a few CT signs (fat, calcification, enhancement) crack almost every case. Master the compartmental approach and the differentials write themselves.

Mediastinal anatomy & compartments

The mediastinum is the space between the two pleural cavities, bounded anteriorly by the sternum, posteriorly by the vertebral column, superiorly by the thoracic inlet, and inferiorly by the diaphragm. For radiological localisation two classification systems matter.

1. Traditional (Felson, plain-film) 4-compartment model — uses lateral chest radiograph landmarks:

Compartment	Boundary (lateral CXR)	Key contents
Superior	Above plane from sternal angle (T4/T5) to lower D4	Thymus, great vessels, trachea, oesophagus
Anterior	Behind sternum, in front of heart & great vessels (anterior to a line along front of trachea + posterior heart border)	Thymus, fat, lymph nodes, ascending aorta
Middle	Heart, pericardium, great vessels, trachea, main bronchi, hila	Heart, vessels, nodes, trachea
Posterior	Behind anterior border of vertebral bodies (technically the paravertebral sulci)	Sympathetic chain, nerves, descending aorta, oesophagus

2. ITMIG / Felson CT-based 3-compartment model (currently preferred, used in modern MCQs):

• Prevascular (anterior): front of pericardium → back of sternum.
• Visceral (middle): between prevascular and a line 1 cm behind the anterior vertebral margin.
• Paravertebral (posterior): paravertebral region behind that line.

High-yield: On a lateral chest radiograph, the anterior mediastinal mass fills the retrosternal clear space; a posterior mass is seen overlying or eroding the vertebrae / posterior ribs. Compartment = the single most useful piece of information.

Compartmental differentials — the master tables

Anterior mediastinum — "The 4 Ts"

This is the NEET PG favourite. The classic mnemonic for an anterior mediastinal mass is the 4 Ts:

High-yield mnemonic — 4 Ts of anterior mediastinum: Thymoma (and thymic lesions) Teratoma (germ cell tumours) Thyroid (retrosternal goitre) Terrible lymphoma (Some add a 5th T — Thymic carcinoma / parathyroid adenoma — "ectopic parathyroid".)

Lesion	Typical age	Key CT/clinical clue
Thymoma	40–60 yrs	Smooth/lobulated soft-tissue mass; myasthenia gravis (~30–50%), red cell aplasia, hypogammaglobulinaemia
Teratoma / GCT	Young adults	Fat + fluid + calcium/tooth = mature teratoma; markers (AFP, βhCG) in malignant GCT
Retrosternal goitre	Middle-aged women	Continuity with cervical thyroid; high attenuation (iodine), avid enhancement, tracheal deviation
Lymphoma	Bimodal (young + elderly)	Lobulated nodal mass, homogeneous, encases vessels without invasion; Hodgkin (nodular sclerosing) classic

Middle mediastinum

Lesion	Clue
Lymphadenopathy (lymphoma, metastatic, TB, sarcoid)	Discrete/conglomerate nodes; eggshell calcification in silicosis/sarcoid
Bronchogenic cyst	Subcarinal, fluid attenuation, does not enhance; may be high HU if proteinaceous
Oesophageal lesions	Duplication cyst, carcinoma, hiatus hernia (air-fluid behind heart)
Vascular	Aortic aneurysm, dilated SVC/azygos
Pericardial cyst	Right cardiophrenic angle, water density, "spring-water" cyst

Posterior mediastinum

Posterior mediastinal masses in any age group are predominantly neurogenic tumours (>70–90%).

Origin	Tumours	Age
Peripheral nerve sheath	Schwannoma (neurilemmoma), Neurofibroma	Adults (20–50)
Sympathetic ganglia	Ganglioneuroma, Ganglioneuroblastoma, Neuroblastoma	Children (neuroblastoma <2 yr)
Paraganglionic	Phaeochromocytoma, chemodectoma	Adults

Other posterior masses: extramedullary haematopoiesis (thalassaemia), lateral meningocoele (NF1), descending aortic aneurysm, Bochdalek hernia, neurenteric cyst.

Age-based approach (a quick triage)

High-yield: Age narrows the list dramatically.

• Child, anterior: lymphoma > teratoma; cystic hygroma extending in.
• Child, posterior: neuroblastoma / ganglioneuroma.
• Adult, anterior: thymoma (most common primary anterior mass in adults), goitre, lymphoma.
• Adult, posterior: schwannoma / neurofibroma.

Overall most common mediastinal mass in adults = neurogenic tumours, while the most common anterior mediastinal mass = thymoma. In children, neurogenic tumours and germ cell/foregut cysts dominate.

CT characteristics — the sign-based approach

CT (with contrast) is the investigation of choice for characterising a mediastinal mass after the chest radiograph. The diagnostic algorithm follows the dominant tissue component:

Stepwise CT approach:

1. Localise the compartment → narrows differential.
2. Assess attenuation:
   • Fat density (−40 to −120 HU) → teratoma, thymolipoma, lipoma, herniated omentum, fat-containing thymus.
   • Fluid density (0–20 HU) → bronchogenic / pericardial / thymic cyst.
   • High density (iodine) → goitre; soft tissue → solid tumour.
3. Look for calcification & its pattern (see below).
4. Assess enhancement → avid (goitre, paraganglioma, Castleman disease, vascular) vs poor (cyst, lymphoma is mildly homogeneous).
5. Check invasion vs displacement → lymphoma encases/displaces vessels; thymic carcinoma & invasive thymoma invade.
6. Continuity → goitre continuous with neck thyroid; neurogenic tumour with neural foramen ("dumb-bell").

Calcification patterns (loved by examiners)

Pattern	Think of
Tooth / chunky / fat-fluid	Mature teratoma
Coarse / curvilinear, mediastinal nodes	Old granulomatous (TB, histoplasma), treated lymphoma
Eggshell nodal calcification	Silicosis, sarcoidosis (less often TB)
Stippled / punctate, posterior, child	Neuroblastoma / ganglioneuroma
Phlebolith-like, well-defined	Vascular malformation
Psammomatous, thyroid	Papillary thyroid carcinoma in goitre

High-yield: Lymphoma rarely calcifies before treatment — calcification in a nodal mass after chemotherapy is a classic clue.

Individual entities — exam essentials

Thymoma

• Most common primary anterior mediastinal tumour in adults (>40 yr); rare under 20.
• Arises from thymic epithelium; round/oval, may have lobulated margin, ±calcification.
• Associations (must know): Myasthenia gravis (commonest), pure red cell aplasia, hypogammaglobulinaemia (Good syndrome), SLE, other autoimmune diseases.
• Masaoka staging (capsular invasion → great vessels) guides prognosis; invasive thymoma "drops" along pleura.
• Treatment: complete surgical resection (DOC) ± radiotherapy for invasive disease.

High-yield: Myasthenia gravis patient + anterior mediastinal mass = thymoma until proven otherwise; conversely ~10–15% of MG patients have a thymoma.

Thymic hyperplasia / rebound

• Diffuse thymic enlargement; rebound hyperplasia after chemotherapy/steroids/illness — must not be mistaken for relapse. Chemical-shift MRI (signal drop on opposed-phase) confirms normal fatty thymus and differentiates hyperplasia from tumour.

Germ cell tumours / teratoma

• Mature (benign) teratoma — fat, fluid, calcium, sometimes teeth/hair → pathognomonic.
• Malignant GCTs (seminoma, non-seminomatous) in young men; raised AFP/βhCG; seminoma is homogeneous and very radiosensitive.

Retrosternal goitre

• Extension of cervical thyroid; key signs: continuity with neck, high baseline attenuation (iodine), intense persistent enhancement, tracheal compression/deviation, calcification.
• Confirmatory: continuity on CT/MRI; radioiodine uptake if functioning.

Lymphoma

• Anterior + middle compartment nodal disease; Hodgkin (nodular sclerosing) classically anterior mediastinal in young adults; non-Hodgkin (large B-cell, lymphoblastic) also.
• Homogeneous lobulated mass, vessel encasement without invasion, low-grade enhancement; PET-CT for staging and response. Diagnosis = biopsy (not surgery first).

Neurogenic tumours

• Posterior, paravertebral; smooth, rounded, may show rib splaying / vertebral scalloping / widened neural foramen.
• Dumb-bell tumour = extension through intervertebral foramen into spinal canal (do MRI to assess cord — investigation of choice for posterior masses).
• Schwannoma/neurofibroma (adult) vs ganglion series (child); neuroblastoma is malignant, calcifies, raised urinary VMA/HVA / catecholamines, MIBG-avid.

High-yield: MRI is the investigation of choice for posterior mediastinal (neurogenic) masses to detect intraspinal (dumb-bell) extension before surgery.

Foregut & mesothelial cysts

• Bronchogenic cyst: subcarinal/paratracheal, water/high attenuation, non-enhancing, no air unless infected/communicating.
• Oesophageal duplication cyst: related to oesophageal wall.
• Pericardial cyst: right cardiophrenic angle, water density, "tear-drop"; benign.

Clinical features

Many mediastinal masses are asymptomatic, found incidentally on imaging. When symptomatic, features arise from mass effect, invasion, or systemic/paraneoplastic syndromes:

• Compressive: cough, dyspnoea, stridor (tracheal), dysphagia (oesophageal), SVC obstruction (facial plethora, distended neck veins, arm oedema — most often lymphoma/bronchogenic carcinoma), hoarseness (recurrent laryngeal nerve), Horner syndrome (sympathetic chain — posterior tumours).
• Systemic/paraneoplastic: myasthenia gravis (thymoma), hypertension/flushing (phaeochromocytoma/paraganglioma), thyrotoxicosis (functioning goitre), B-symptoms (lymphoma), gynaecomastia (βhCG-secreting GCT).

High-yield: Malignant masses are more likely symptomatic; the probability of malignancy is higher in children and in symptomatic adults.

Diagnosis & investigation of choice

Imaging ladder:

1. Chest radiograph (PA + lateral) → first test; localises compartment, silhouette sign, hilum overlay sign, cervicothoracic sign (to tell anterior from posterior at thoracic inlet).
2. Contrast-enhanced CT chest → investigation of choice overall for characterisation (attenuation, fat, calcium, enhancement, invasion).
3. MRI → best for posterior/neurogenic masses (cord, dumb-bell), cyst vs solid, chest-wall/vascular invasion; chemical-shift for thymic hyperplasia.
4. Tumour markers: AFP, βhCG (GCT); thyroid function/scan (goitre); urinary catecholamines/VMA (neural).
5. PET-CT → lymphoma staging, thymic malignancy.
6. Tissue: CT-guided core biopsy / mediastinoscopy / EBUS for nodal disease; avoid up-front surgery for suspected lymphoma (medical treatment).

High-yield useful CXR signs:

• Cervicothoracic sign: a mass whose upper margin fades above the clavicles is posterior; if the sharp border is lost at the clavicle level it lies anterior.
• Hilum overlay sign: hilar vessels seen through the mass → mass is not in the middle mediastinum (anterior or posterior).
• Silhouette sign localises which structure the mass abuts.

Management / drug & treatment of choice (overview)

• Thymoma / thymic carcinoma: complete surgical resection ± adjuvant RT/chemo (invasive). Thymectomy also benefits MG.
• Mature teratoma: surgical excision (curative). Malignant GCT: cisplatin-based chemotherapy (seminoma very chemo/radiosensitive).
• Lymphoma: chemotherapy ± radiotherapy (ABVD for Hodgkin; R-CHOP for DLBCL) — not primary surgery.
• Retrosternal goitre: surgery if compressive/suspicious; thyroxine/radioiodine in selected cases.
• Neurogenic tumours: surgical excision; neuroblastoma per risk-stratified multimodal protocol; phaeochromocytoma needs alpha-blockade (phenoxybenzamine) before surgery.
• Bronchogenic/pericardial cysts: excision if symptomatic; otherwise observe.

Complications

• SVC syndrome, airway obstruction, recurrent laryngeal/phrenic nerve palsy, Horner syndrome.
• Pleural/pericardial effusion, cardiac tamponade.
• Cyst infection, rupture, haemorrhage; teratoma rupture into airway → expectorating hair/oil.
• Spinal cord compression from dumb-bell tumours.
• Malignant transformation/invasion; metastatic spread.
• Anaesthetic crisis: a large anterior mass can cause airway/great-vessel collapse on induction (supine) — assess carefully before GA.

Key differentials & distinguishing pearls

Confusing pair	Discriminator
Thymoma vs lymphoma (anterior)	Thymoma older, focal, MG; lymphoma younger, multinodal, B-symptoms, no calcification pre-Rx
Thymic hyperplasia vs tumour	Chemical-shift MRI signal drop = benign fatty hyperplasia
Bronchogenic cyst vs solid node	Cyst = fluid HU, non-enhancing, smooth, subcarinal
Goitre vs other anterior mass	Continuity with neck + high attenuation + avid enhancement
Schwannoma vs neuroblastoma	Adult, benign, foraminal vs child, calcified, raised catecholamines
Aneurysm vs solid mass	Aneurysm enhances like aorta, continuous lumen — never biopsy!

Recently asked / exam angle

• "4 Ts of anterior mediastinum" — direct single-best-answer recall (Thymoma, Teratoma, Thyroid, Terrible lymphoma). Recognise the variant adding parathyroid/thymic carcinoma.
• Image-based: lateral CXR with retrosternal opacity → anterior compartment → list 4 Ts.
• Fat + calcification (tooth) on CT → mature teratoma.
• Posterior mediastinal mass with widened neural foramen / dumb-bell → neurogenic tumour; MRI is the next investigation.
• Myasthenia gravis + anterior mass → thymoma; reverse association also asked.
• Most common posterior mediastinal mass = neurogenic tumour; most common in children, posterior = neuroblastoma.
• Eggshell calcification of nodes → silicosis/sarcoidosis.
• Cervicothoracic sign / hilum overlay sign → localisation MCQs.
• Investigation of choice: CECT overall; MRI for posterior/neural lesions; chemical-shift MRI for thymic rebound.
• Subcarinal bronchogenic cyst as the commonest foregut duplication cyst.

Rapid revision

1. Anterior mediastinal mass = 4 Ts — Thymoma, Teratoma, Thyroid (goitre), Terrible lymphoma.
2. Thymoma = commonest primary anterior mediastinal tumour in adults; links to myasthenia gravis, red cell aplasia.
3. Posterior mediastinal mass = neurogenic tumour (commonest overall mediastinal mass in adults).
4. Neuroblastoma = commonest posterior mass in young children; raised VMA/HVA, MIBG-avid, calcifies.
5. Mature teratoma = fat + fluid + calcium/tooth on CT — pathognomonic.
6. CECT chest is the investigation of choice; MRI is best for posterior/dumb-bell and cord assessment.
7. Retrosternal goitre shows continuity with neck thyroid, high attenuation, intense enhancement.
8. Lymphoma encases vessels, is homogeneous, and does not calcify before treatment.
9. Eggshell nodal calcification → silicosis / sarcoidosis.
10. Cervicothoracic sign distinguishes anterior from posterior masses at the thoracic inlet; hilum overlay sign excludes a middle-mediastinal mass.
11. Bronchogenic cyst = subcarinal, fluid density, non-enhancing.
12. Never biopsy a suspected aortic aneurysm; chemical-shift MRI confirms benign thymic rebound hyperplasia after chemotherapy.