2  Mediastinal Mass

2.1 Anterior Mediastinal Masses

The anterior mediastinum is the space between the sternum and a line anterior to the heart and great vessels. Masses in this compartment have characteristic features and a specific differential diagnosis.

2.1.1 The “Four Ts” Mnemonic for Anterior Mediastinal Masses

The classic mnemonic for anterior mediastinal masses is the “Four Ts”:

  • Thymoma (most common anterior mediastinal mass in adults) and other thymic lesions
  • Thyroid lesions (goiter, adenoma, carcinoma)
  • T-cell lymphoma (Hodgkin lymphoma and Non-Hodgkin lymphoma)
  • Teratoma and other germ cell tumors (seminoma, choriocarcinoma)

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2.1.2 Detailed Differential Diagnosis

2.1.2.1 Thymic Masses

  • Thymoma: Most common anterior mediastinal tumor in adults (rare in children)
    • Benign thymoma (75%)
    • Malignant thymoma (25%) - invasive, can have pleural drop metastases
    • Associated with myasthenia gravis (35% of thymoma patients have MG; 15% of MG patients have thymoma)
    • Other associations: aplastic anemia, hypogammaglobulinemia, red cell aplasia
    • Imaging: anterior mediastinal mass, often asymmetric, may have calcifications (20%)
  • Thymic Hyperplasia
    • Diffuse symmetric enlargement without focal mass
    • Seen in myasthenia gravis, thyrotoxicosis, Graves disease, collagen vascular diseases
    • Rebound hyperplasia after chemotherapy, Addison disease, acromegaly
  • Thymic Cyst
    • Third pharyngeal pouch remnant
    • Homogeneous cystic mass
  • Thymolipoma
    • Benign, encapsulated tumor containing both thymic and adipose tissue
    • Occurs in children and young adults
    • Usually grows to large sizes (75% weigh >500g)
    • Associated with myasthenia gravis, aplastic anemia, Graves disease
  • Thymic Carcinoma/Carcinoid
    • Rare, aggressive thymic neoplasms

[Core_Thoracic_Imaging, 57-59] [Primer_Chest Imaging, 58-59]

2.1.2.2 Thyroid Masses

  • Goiter: Most common thyroid mass extending into mediastinum
    • Thoracic inlet mass continuous with cervical thyroid
    • Heterogeneous density on CT with calcium, iodine, colloid cysts
    • Marked contrast enhancement
    • May cause tracheal deviation
    • Usually anterior to brachiocephalic vessels (80%)
  • Thyroid Carcinoma
    • Less common
    • Irregular borders on imaging

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2.1.2.3 Lymphoma

  • Hodgkin Lymphoma
    • Bimodal age distribution with peaks at 30 and 70 years
    • Usually involves superior mediastinal nodes
    • Contiguous progression from one lymph node group to the next
    • Types: lymphocyte predominant, nodular sclerosing (most common, 70%), mixed cellularity, lymphocyte depleted
  • Non-Hodgkin Lymphoma
    • 85% arise from B cells, 15% from T cells
    • Often generalized at presentation
    • Adenopathy may be noncontiguous
    • Affects all age groups (mean age 50)
    • Increased incidence in immunocompromised patients

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2.1.2.4 Germ Cell Tumors

  • Teratoma (70% of germ cell tumors)
    • Most common, 20% are malignant
    • Variable tissue contents: calcification (30%), fat, cystic areas, soft tissue
    • Fat-fluid levels may be present
  • Seminoma
    • Most common malignant germ cell tumor
    • Large, unencapsulated lesions
    • Almost exclusively in men 10-39 years old
    • May be associated with testicular atrophy
  • Embryonal Cell Carcinoma
    • Mediastinal invasion is common
    • Poor prognosis
    • Elevated α-fetoprotein and human chorionic gonadotropin levels
  • Choriocarcinoma
    • Rare, highly aggressive

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2.1.3 Additional Anterior Mediastinal Masses (Less Common)

  • Aneurysm of ascending aorta
  • Pericardial cyst
  • Lymphangioma
  • Fibrosing mediastinitis

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2.1.4 Imaging Characteristics of Anterior Mediastinal Masses

2.1.4.1 Radiographic Features

  • Displaced anterior junction line
  • Obliterated cardiophrenic angles
  • Obliterated retrosternal clear space
  • Hilum overlay sign (hilar vessels visible through the mass)
  • Effacement/dense ascending aorta

2.1.4.2 Features Suggesting Fat Content

  • Thymolipoma
  • Teratoma
  • Lipoma
  • Liposarcoma

2.1.4.3 Features Suggesting Cystic Content

  • Thymic cyst
  • Cystic thymoma
  • Teratoma (cystic with enhanced septations)
  • Pericardial cyst

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