Anterior Compartment Mass

Robert B. Carr, MD

DIFFERENTIAL DIAGNOSIS

Common

Lymphoma
Germ Cell Tumor
- Teratoma
- Seminoma
Thyroid Mass
Thymoma
Thymic Hyperplasia
Lipomatosis
Metastasis

Less Common

Thymic Mass
- Thymic Carcinoma
- Thymic Carcinoid
- Thymic Cyst
Parathyroid Mass

Rare but Important

Thymolipoma
Lymphangioma
Nonseminomatous Germ Cell Tumor

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Anatomy of anterior compartment
- Radiologic description is based upon radiographic landmarks as defined by Fraser and Pare; note this differs from surgical description
- Anterior border is sternum, and posterior border is anterior margin of vertebral column
Normal contents: Thymus, ascending aorta, great vessels, part of main pulmonary artery, heart, pericardium, lymph nodes, adipose tissue
CT is invaluable for determining site of origin and tissue characterization

Helpful Clues for Common Diagnoses

Lymphoma
- Hodgkin disease (HD) more common than non-Hodgkin lymphoma (NHL) within anterior compartment
- Enlarged lymph nodes or nodal mass, usually displaying homogeneous soft tissue attenuation
- Necrosis is occasionally present, usually detected after contrast administration
- HD commonly involves several contiguous nodal groups in thorax
- Involvement of single nodal group is more common with NHL
Germ Cell Tumor
- Most common age is 2nd-4th decades
- More than 80% are benign
- Teratoma
  - Most common benign germ cell tumor, though can be malignant
  - Display cystic areas, soft tissue, fat, and calcification
  - Fat-fluid level is diagnostic but usually not present
- Seminoma
  - Most common malignant germ cell tumor and usually in males
  - Large homogeneous mass, which can have small focal areas of decreased attenuation
Thyroid Mass
- Caused by inferior extension of thyroid lesion
- Direct connection to thyroid is usually evident on CT
- High attenuation on noncontrast CT is due to iodine content
- Differentiation of goiter and tumor may not be possible
Thymoma
- Most common in 6th decade
- Associated with myasthenia gravis
- Round or lobulated and usually homogeneous
- Possible areas of necrosis, hemorrhage, calcification, and cyst formation
- Does not conform to normal thymic contour and may be unilateral
- Classified as invasive or noninvasive based upon invasion of adjacent structures (including vessels, heart, and pericardium); determination not always possible with CT
- Pleural spread often produces multiple pleural implants
Thymic Hyperplasia
- Associated with recovery from chemotherapy or burn (thymic rebound) in children and young adults
- Associated with Grave disease, myasthenia gravis, red cell aplasia, and other conditions in adults
- Thymic rebound often visible on chest radiograph; correlate with clinical history
- Enlarged thymus with normal homogeneous attenuation on CT
Lipomatosis
- Excessive unencapsulated fat in mediastinum associated with Cushing syndrome, steroids, obesity
- Smooth, symmetric mediastinal widening on chest radiograph
- Homogeneous increased amount of mediastinal fat with smooth margins on CT
Metastasis
- Lung and breast primaries are common
- May involve thymus or mediastinal lymph nodes
- Appearance is nonspecific

Helpful Clues for Less Common Diagnoses

Thymic Carcinoma
- Large mass that can have areas of necrosis
- Similar to thymoma in appearance, but distant metastases are far more common than with thymoma
- Metastases often involve lungs, skeleton, liver, and brain
Thymic Carcinoid
- Commonly secretes ACTH, which results in Cushing syndrome
- Appears similar to thymoma on imaging, though typically more aggressive
Thymic Cyst
- Care should be taken not to confuse with cystic neoplasm
- Nonenhancing, thin-walled, water density, no soft tissue component
Parathyroid Mass
- Normal glands are not visible on CT
- Ectopic glands often found in thymic bed
- Cannot distinguish adenoma, hyperplasia, and carcinoma on CT

Helpful Clues for Rare Diagnoses

Thymolipoma
- Most common age is 1st-4th decades
- Usually asymptomatic and large at time of detection
- May appear to drape over heart on chest radiograph
- Primarily fat density on CT with strands of soft tissue attenuation
Lymphangioma
- Usually congenital and often presents in childhood
- Well circumscribed with homogeneous water attenuation
- May wrap around mediastinal structures, such as great vessels
Nonseminomatous Germ Cell Tumor
- Aggressive neoplasms with poor prognoses
- Infiltrative and heterogeneous with areas of hemorrhage and necrosis