Mediastinal Mass: A Comprehensive Guide for MCCQE1

Introduction

The evaluation of a mediastinal mass is a high-yield topic for the MCCQE1 and requires a structured approach rooted in anatomy. As a Canadian medical student, you must demonstrate the CanMEDS role of a Medical Expert by correlating the anatomical location of the mass with the patient’s age and clinical presentation to generate a differential diagnosis.

The mediastinum is the central compartment of the thoracic cavity. It is bounded by the sternum anteriorly, the vertebral column posteriorly, and the lungs laterally.

Anatomical Compartments and Differential Diagnosis

For clinical purposes and MCCQE1 preparation, the mediastinum is divided into three compartments: Anterior, Middle, and Posterior.

Differential Diagnosis by Compartment

Clinical Presentation

Approximately 30-50% of patients are asymptomatic, and the mass is an incidental finding on imaging. When symptoms occur, they are typically due to the compression or invasion of adjacent structures.

Local Mass Effects

Systemic Syndromes (Paraneoplastic)

• Myasthenia Gravis: Strongly associated with Thymoma.
• Hypercalcemia: Parathyroid adenoma or bone metastasis.
• Hypoglycemia: Solitary fibrous tumours (production of IGF-II).
• Gynecomastia: Germ cell tumours ($\beta$-hCG production).

Diagnostic Approach

Follow this stepwise approach to maximize your score on MCCQE1 Clinical Decision Making (CDM) stations.

// Tumour Marker Panel
const markers = {
  AFP: "Alpha-fetoprotein", // Elevated in Non-seminomatous GCT
  bHCG: "Beta-human chorionic gonadotropin", // Elevated in Seminoma & Non-seminomatous
  LDH: "Lactate Dehydrogenase" // Nonspecific, elevated in Lymphoma/high turnover
};

Management Principles

Management depends entirely on the etiology.

1. Thymoma: Surgical resection (Thymectomy). Even benign thymomas are resected due to invasion potential.
2. Lymphoma: Medical management (Chemotherapy/Radiation). Surgery is generally not indicated except for biopsy.
3. Germ Cell Tumours:
   • Seminoma: Highly radiosensitive and chemosensitive.
   • Non-seminomatous: Chemotherapy followed by resection of residual mass.
   • Teratoma: Surgical resection.
4. Neurogenic Tumours: Surgical resection.
5. Goitre: Thyroidectomy (often feasible via cervical approach).

Canadian Guidelines & Epidemiology

Epidemiology

• Age Matters:
  • Children: Neurogenic tumours (posterior) are the most common.
  • Adults: Thymomas (anterior) are the most common primary mediastinal mass.
• Lymphoma: Hodgkin’s lymphoma has a bimodal distribution (young adults and elderly).

Guidelines (Cancer Care Ontario / BC Cancer)

• Referral: Any patient with a suspected mediastinal malignancy should be referred to a Thoracic Surgeon or a Multidisciplinary Tumor Board (MDT).
• PET Scans: In Canada, PET/CT is funded and indicated for the staging of Lymphoma and investigation of solitary pulmonary nodules/mediastinal masses when curative intent is possible.
• Biopsy Safety: Canadian guidelines emphasize avoiding percutaneous biopsy in potentially curable thymomas if the capsule might be breached, risking seeding. Surgical excision is preferred if clinical suspicion is high.

Key Points to Remember for MCCQE1

• Anterior mass + Young Male = Germ Cell Tumour until proven otherwise. Order AFP and $\beta$-hCG.
• Anterior mass + Myasthenia Gravis = Thymoma.
• Posterior mass + Child = Neuroblastoma.
• Middle mass + Weight loss + Night Sweats = Lymphoma or TB.
• Never biopsy a testicular mass; perform radical orchiectomy. However, for a mediastinal primary GCT, biopsy is done if markers are negative.
• EBUS is the procedure of choice for sampling mediastinal lymph nodes (middle mediastinum).

Sample Question

Explanation

The correct answer is:

• C. Non-seminomatous germ cell tumour

Detailed Analysis

• Analysis of the Stem:

  • Patient: 26-year-old male (Young male demographic).
  • Location: Anterior mediastinal mass.
  • Key Finding: Significantly elevated AFP and $\beta$-hCG.

• Reasoning:

  • C. Non-seminomatous germ cell tumours (NSGCT): These include embryonal carcinoma, yolk sac tumour, choriocarcinoma, and mixed tumours. They are characteristically associated with elevated AFP and/or $\beta$-hCG. The combination of a large anterior mediastinal mass in a young male with elevated AFP is diagnostic of an NSGCT (specifically with a yolk sac component for AFP).
  • A. Seminoma: While seminomas are common germ cell tumours, they typically produce elevated $\beta$-hCG (in about 10-30% of cases) but never elevate AFP. If AFP is elevated, the diagnosis is Non-seminomatous GCT.
  • B. Thymoma: Thymomas are epithelial tumours. They do not secrete AFP or $\beta$-hCG. They are often associated with autoimmune conditions like Myasthenia Gravis.
  • D. Hodgkin’s lymphoma: A common cause of anterior mediastinal masses in young adults. However, it does not result in elevated AFP or $\beta$-hCG. LDH may be elevated, but it is nonspecific.
  • E. Substernal thyroid goitre: This would present as a mass continuous with the thyroid gland in the neck. It does not secrete AFP or $\beta$-hCG.

References

1. Medical Council of Canada. (2023). MCCQE Part I Clinical Decision-Making and Multiple-Choice Question Objectives.
2. Kasper, D. L., et al. (2022). Harrison’s Principles of Internal Medicine, 21st Edition. McGraw-Hill Education.
3. Cancer Care Ontario. (2023). Pathology Reporting of Resection Specimens for Thymoma and Thymic Carcinoma.
4. Carter, B. W., et al. (2014). ITMIG Classification of Mediastinal Compartments and Multidisciplinary Approach to Mediastinal Masses. RadioGraphics.
5. Toronto Notes. (2024). Respirology & Thoracic Surgery Chapters. Toronto Notes for Medical Students, Inc.