# Mediastinal Pathology

> Mediastinal pathology for MBBS Pathology: anterior middle posterior compartment masses, thymoma with myasthenia, germ cell tumours and neurogenic tumours.

- Canonical URL: https://prepelephant.com/topics/mbbs/pathology/mediastinal-pathology
- Exam / course: MBBS · Subject: Pathology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Mediastinal Pathology", PrepElephant, https://prepelephant.com/topics/mbbs/pathology/mediastinal-pathology

## 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).