Which is the most common primary mediastinal mass in adults?

Explanation

## Mediastinal Masses — Most Common Primary Mass in Adults **Key Point:** Neurogenic tumours are the most common primary mediastinal masses in adults overall, accounting for approximately 20–25% of all mediastinal masses across all compartments. They arise predominantly in the posterior mediastinum. ### Distribution by Compartment and Frequency | Mass Type | Mediastinal Compartment | Frequency (All Adults) | |-----------|------------------------|------------------------| | Neurogenic tumour | Posterior | Most common overall (~20–25%) | | Thymoma | Anterior | Most common *anterior* mass (~40–50% of anterior masses) | | Lymphoma | Anterior/Middle | Common | | Germ cell tumour | Anterior | Less common | **High-Yield:** The distinction is critical for exam purposes: - **Most common primary mediastinal mass overall in adults → Neurogenic tumour** (posterior mediastinum) - **Most common anterior mediastinal mass in adults → Thymoma** ### Why Neurogenic Tumours? Neurogenic tumours include schwannomas, neurofibromas, and ganglioneuromas. They arise from intercostal nerves, sympathetic ganglia, or the vagus nerve in the posterior mediastinum. Per Harrison's Principles of Internal Medicine and standard surgical texts (Shields' General Thoracic Surgery), neurogenic tumours collectively represent the single largest category of primary mediastinal neoplasms in adults when all compartments are considered together. **Mnemonic — Posterior Mediastinal Masses (3 Ns):** - **N**eurogenic tumours (most common) - **N**odal masses (lymphoma) - **N**ot-so-common (oesophageal, vascular) ### Clinical Features of Neurogenic Tumours - Often asymptomatic; discovered incidentally on chest X-ray or CT - May cause back pain, intercostal neuralgia, or Horner syndrome (if apical) - "Dumbbell" tumours extend through intervertebral foramina into the spinal canal - Malignant peripheral nerve sheath tumours (MPNSTs) are rare but aggressive **Clinical Pearl:** A posterior mediastinal mass on imaging in an adult should raise suspicion for a neurogenic tumour. MRI is the preferred modality to assess intraspinal extension ("dumbbell" configuration). **Warning:** Do not confuse "most common anterior mediastinal mass" (thymoma) with "most common primary mediastinal mass overall" (neurogenic tumour). Many exam questions exploit this distinction — read the stem carefully. ### Anatomical Basis The posterior mediastinum is bounded by: - **Anterior:** Pericardium and great vessels - **Posterior:** Vertebral column - **Lateral:** Mediastinal pleura - **Inferior:** Diaphragm The rich neural structures in this compartment (intercostal nerves, sympathetic chain, vagus) explain the predominance of neurogenic tumours here. *Reference: Shields TW. General Thoracic Surgery, 7th ed.; Harrison's Principles of Internal Medicine, 20th ed.*