Low-Attenuation Mass, Mediastinum or Hilum

Jud W. Gurney, MD, FACR

DIFFERENTIAL DIAGNOSIS

Common

Diaphragmatic Hernia
Lipomatosis
Lipoma

Less Common

Low-Attenuation Lymph Nodes
- Mediastinal Metastases
- Infection: Fungal & Tuberculosis
Nerve Sheath Tumors
Mediastinal Abscess
Thymolipoma
Teratoma (Dermoid Cyst)
Mediastinal Cyst
Liposarcoma
Lymphangioma
Hemangioma
Thymic Cyst

Rare but Important

Mediastinal Pseudocyst
Lateral Meningocele
Epipericardial Fat Pad Necrosis
Extramedullary Hematopoiesis
Whipple Disease (Intestinal Lipodystrophy)

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Fat (-70 to -130 HU) vs. fluid (0-30 HU)
Mnemonic for fat-containing lesions: LITHE (yes, this is an oxymoron)
- Lipomatosis, lipoma, liposarcoma
- Intestinal lipodystrophy
- Thymolipoma, teratoma (mature)
- Hernias, hemangioma
- Extramedullary hematopoiesis, epipericardial fat pad necrosis
Mnemonic for water density lesions: FLUIDS
- Foregut duplication cysts, lymphangioma, pseudocyst, infection (nodes and abscess), desmoid, spine (meningocele)
Enlarged normal lymph nodes often have central fat or fatty hilum

Helpful Clues for Common Diagnoses

Diaphragmatic Hernia
- Includes hiatal hernia, Bochdalek, Morgagni, and traumatic hernias
- Contents typically include fat and bowel
Lipomatosis
- Location: Upper mediastinum, costophrenic angles, paraspinal
- Unencapsulated fat
- Associated with generalized obesity, Cushing disease, corticosteroid therapy
Lipoma
- Location: Typically anterior mediastinum
- Encapsulated, may be pedunculated
- Well-marginated, fat only
  - Any soft tissue component, consider liposarcoma or thymolipoma

Helpful Clues for Less Common Diagnoses

Low-Attenuation Lymph Nodes
- Mediastinal Metastases
  - Metastases may be low attenuation from necrosis or cystic degeneration
  - Typical tumors include bronchogenic carcinoma, testicular, ovarian, and treated lymphoma
- Infection: Fungal & Tuberculosis
  - Enlarged nodes with rim enhancement and low-attenuation centers
  - Indicates active disease
Nerve Sheath Tumors
- Neurofibroma or schwannoma
- Frequent low attenuation (15-20 HU) due to lipid content or cystic degeneration
Mediastinal Abscess
- Descending cervical mediastinitis usually from odontogenic or cervicofacial infection, esophageal perforation, or trauma
- Caudal spread aided by gravity and negative intrapleural pressure
- Irregularly shaped fluid collections, may contain air
Thymolipoma
- Anterior mediastinal mass, conforms to shape of adjacent structures
- Typically large; mean length: 18 cm
- Tumor contains mixture of fat (at least 50%) and soft tissue
  - Soft tissue seen as linear strands or whorls, uncommonly rounded nodules
Teratoma (Dermoid Cyst)
- Anterior mediastinal mass
- Fat in 75%, fluid in 90%
- Mixture of fat, soft tissue, fluid, and calcification (50%)
- Cystic component often predominant (multilocular), 15% cystic only
Mediastinal Cyst
- Includes foregut duplication cysts, pericardial cysts
- Cysts are thin walled, unilocular
- Fluid attenuation variable: Water, hemorrhage, infection, milk of calcium
- Bronchogenic cysts usually subcarinal; esophageal duplication cysts periesophageal; neurenteric cysts associated with adjacent vertebral body cleft
Liposarcoma
- Location: Typically posterior mediastinum
- Inhomogeneous with large areas of soft tissue density
Lymphangioma
- Multilocular, well-defined, water density mass; may be septated
- Location: Superior mediastinum adjacent to right lateral tracheal wall
- Soft in composition, no mass effect
- Intrathoracic lymphangiomas + cystic bone lesions = Gorham disease
Hemangioma
- Location: Superior mediastinum
- Fat in 40%, phleboliths in 10-40%
Thymic Cyst
- Congenital cysts most common, usually unilocular
- Acquired cysts usually multilocular
  - Occurs in patients after radiation therapy for Hodgkin disease, in association with thymic tumors, and after thoracotomy

Helpful Clues for Rare Diagnoses

Mediastinal Pseudocyst
- Pancreatic pseudocyst extending through esophageal or aortic hiatus
- Location: Posterior inferior mediastinum
- Fluid collection, thin or thick walled
- Usually connects to pancreatic pseudocysts
Lateral Meningocele
- Associated with neurofibromatosis
- May be multiple and bilateral
- Typically enlarges neural foramen
Epipericardial Fat Pad Necrosis
- Patients usually present with acute pleuritic chest pain
- Imaging and pathologic features similar to those of fat necrosis in epiploic appendagitis
Extramedullary Hematopoiesis
- Typically in patients with congenital hereditary anemias, especially thalassemia
- Posterior mediastinal masses usually caudal to 6th thoracic vertebra
- May contain fat, especially larger lesions
- Centered on vertebral body with prominent trabeculae from marrow expansion
Whipple Disease (Intestinal Lipodystrophy)
- Infection caused by Tropheryma whippelii
- Migratory polyarthritis followed by intestinal malabsorption
- Low-density nodes from foamy lipid-containing macrophages

Image Gallery

Axial CECT shows a hiatal hernia of peritoneal fat tissue through esophageal hiatus

. Note the sparse linear vessels

typical for herniated abdominal fat.

Axial CECT shows Bochdalek hernia containing retroperitoneal fat

. Note the localized discontinuity of the medial left hemidiaphragm

(Left) Axial CECT shows herniation of peritoneal fat

through Morgagni hiatus. Morgagni hernias are typically right-sided; the left side is blocked by the heart. (Right) Coronal CECT shows large right pleural effusion

, small bowel loops

, and peritoneal fat

from traumatic diaphragmatic tear. Right-sided tears are less common than tears of the left hemidiaphragm. Coronal reconstructions are often useful for diaphragmatic hernias.

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