Mediastinal Pathology
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Direct answer
Compartment, then diagnosis: an anterior mediastinal mass in an adult is one of the four Ts — thymoma, teratoma (germ cell tumour), thyroid enlargement (retrosternal goitre) or "terrible lymphoma" — while middle mediastinal masses are cysts, lymphadenopathy and bronchogenic tumours, and posterior mediastinal masses in a young patient are neurogenic tumours until proved otherwise. Thymoma carries the examination's favourite systemic links: myasthenia gravis in a substantial minority of patients (and thymic hyperplasia with germinal centres in most myasthenics), pure red cell aplasia and hypogammaglobulinaemia. In India, tuberculosis produces middle-compartment lymphadenopathy that easily mimics lymphoma, so the mediastinal mass work-up always keeps both in the differential.
What you must remember
- Anterior compartment (behind sternum, above heart): thymoma, germ cell tumours (teratodermoid most common, seminoma, embryonal carcinoma, yolk sac with elevated alpha-fetoprotein, choriocarcinoma with beta-hCG), retrosternal thyroid, lymphoma; symptomatic anterior masses are malignant more often than incidental ones.
- Middle compartment: pericardial and bronchogenic cysts, lymph node disease — tuberculosis and sarcoidosis in India, metastases, and lymphoma — plus tracheal tumours.
- Posterior compartment (paravertebral gutter): schwannoma and neurofibroma from nerve sheath, ganglioneuroma and neuroblastoma from sympathetic ganglia (children), paraganglioma; neuroblastoma in a child under five with elevated urinary vanillylmandelic acid and homovanillic acid is the classic.
- Thymoma histology and staging: epithelial tumour with a variable lymphocytic component classified WHO A, AB, B1-B3 and C (thymic carcinoma); Masaoka staging by capsular invasion determines prognosis; encapsulated tumours are curable by excision.
- Thymoma's systemic triad: myasthenia gravis, pure red cell aplasia, and acquired hypogammaglobulinaemia (Good syndrome with recurrent infections) — the exam expects all three, not just the first.
- Teratoma rule: anterior mediastinal germ cell tumours in men over 50 with elevated markers behave as malignant non-seminomatous tumours; a dermoid cyst containing hair, teeth and sebum in a young adult is the benign exception that still needs excision.
- Emergency associations: superior vena cava syndrome from lymphoma or small cell lung carcinoma; myasthenic crisis; and spinal cord compression from a posterior dumbbell tumour extending through the intervertebral foramen.
A case that shows why biopsy order matters
A 42-year-old man has retrosternal discomfort on a chest radiograph showing an anterior mediastinal mass with a lobulated contour. The sequence matters. Step one: compute the compartment — anterior, confirmed on contrast tomography — so thymoma, germ cell tumour, thyroid and lymphoma lead the list. Step two: tumour markers — alpha-fetoprotein and beta-hCG to exclude a non-seminomatous germ cell tumour, since a markedly elevated AFP in an anterior mass may go straight to chemotherapy rather than the operating table. Step three: myasthenia screen, because ptosis and fatigability discovered preoperatively change anaesthetic planning (avoid non-depolarising blockers and over-sedation). Step four: excision for an encapsulated-appearing lesion — the pathologist reports a WHO B2 thymoma, Masaoka stage II with microscopic capsular invasion, and the surgeon completes the resection. Compare the counter-case of a 24-year-old with a smooth posterior mass and scoliosis: a schwannoma growing through a foramen as a dumbbell, imaged with magnetic resonance before excision to spare the cord. Same radiograph finding, opposite ends of the mediastinum, entirely different conversations.
Where students slip
The recurring slip is anatomical: candidates call any mass behind the sternum "anterior mediastinal" without knowing the boundaries — anterior compartment between sternum and pericardium, middle containing pericardium and heart, posterior behind the pericardium to the vertebrae — and lose the diagnostic shortlist built on them. The second error is forgetting that in Indian practice the commonest middle-mediastinal mass is tuberculous lymphadenopathy, so a necrotic confluent nodal mass on tomography triggers sputum, GeneXpert and endoscopic biopsy rather than an immediate lymphoma protocol. The third, a viva favourite: distinguishing thymoma from thymic hyperplasia — the myasthenic's thymus shows lymphoid follicles with germinal centres in otherwise preserved architecture, whereas thymoma is a neoplastic epithelial proliferation; confusing the two misdirects both surgery and staging.
Frequently asked questions
What are the commonest tumours of each mediastinal compartment?
Anterior — thymoma, germ cell tumour, retrosternal thyroid and lymphoma; middle — cysts and lymphadenopathy (tuberculous in India); posterior — neurogenic tumours such as schwannoma.
Which systemic diseases associate with thymoma?
Myasthenia gravis most often, plus pure red cell aplasia and hypogammaglobulinaemia with recurrent infections (Good syndrome).
Why check AFP and beta-hCG in an anterior mediastinal mass?
Eleved markers identify non-seminomatous germ cell tumours, which are treated with cisplatin-based chemotherapy rather than primary surgery.
What is the classic posterior mediastinal tumour of childhood?
Neuroblastoma of the sympathetic chain, diagnosed under five years of age with a paravertebral mass and raised urinary vanillylmandelic and homovanillic acids.
What does Masaoka staging of thymoma depend on?
Anatomical invasion — from an intact capsule (stage I) through microscopic capsular breach (II), adjacent-organ invasion (III), pleural or pericardial spread (IVa) and distant metastases (IVb).