Radiography
Findings
Many thymomas may be visualized using routine chest radiographs in posteroanterior (PA) and lateral views. A thymoma often appears as a smooth lobulated mass in the superior aspect of the anterior mediastinum, often projecting into one of the hemothoraces. The mass may be calcified or cystic (see Images 4, 9, 11).
In most cases, thymolipomas are detected incidentally on chest radiographs. They are often large, ranging up to 36 cm in diameter (mean, 18 cm) at the time of diagnosis. They may project into the hemithoraces; they may be seen draping over the heart and extending as far as the costophrenic angles. They can be easily mistaken for pleural or pericardial tumors or even pulmonary sequestration.
Computed Tomography
Findings
From birth to puberty, CT attenuation of the thymus is comparable to that of the chest wall musculature. The outer contours of the thymus may be convex laterally. The lobes, though separate, may have a triangular shape and may be slightly rotated to the left. The junction between the lobes is about 2 cm to the left of the midline (see Images 3, 5-8, 10, 12).
From puberty to 30 years age, the overall attenuation value diminishes secondary to fatty infiltration; during this period, attenuation is less than that of skeletal muscle. The thymus is often seen as a discrete triangular or bilobed structure, with the outer borders appearing straight or slightly concave laterally.
After the age of 30 years, remnants of thymic tissue appear as small islands of soft tissue attenuation; these islands appear as linear, oval, or small and round configurations on a background of more abundant fat.
In the older patient, only a thin fibrous skeleton of the thymus remains. At this point, the thymus is almost totally composed of fat.
The most reliable and meaningful measurements of the thymus are related to its thickness. Before the age of 20 years, 1.8 cm is the normal thickness; thereafter, the normal thymus does not exceed 1 cm in thickness.
Thymic hyperplasia
CT may show symmetric, diffuse enlargement of the thymus. When the enlargement is asymmetric, it may mimic a thymoma; thymoma should be considered in the differential diagnosis.
Mature teratoma
On CT, teratomas are sharply defined; low-attenuation cystic components predominate. Fatty tissue is seen in about 50% of cases, and fat-fluid levels have been reported. Foci of calcification and ossification are commonly seen; in addition, soft tissue attenuation may be depicted. In rare cases, the tumor may erode and rupture into adjacent structures, such as the lung, tracheobronchial tree, and pleural space.
Seminoma
On CT, seminomas are usually large and homogeneous, with soft tissue attenuation. Areas of low attenuation often are present secondary to necrosis and hemorrhage.
Nonseminomatous tumor
Nonseminomatous tumors include embryonal carcinoma, endodermal sinus tumor, choriocarcinoma, and mixed germ cell tumor. On CT, these lesions are often large and heterogeneous, with large (>50%) areas of low attenuation. They may contain areas of calcification.
Thymic lymphoma
On CT, the thymus is enlarged, symmetrically or asymmetrically; this enlargement often makes it difficult to differentiate the thymus from the enlarged surrounding lymph nodes. At times, differentiating the thymus from a thymoma is difficult, though the clinical picture and the presence of other sites of lymphadenopathy are often helpful in the diagnosis.
Thymic carcinoma
Thymic carcinoma belongs to a group of uncommon epithelial neoplasms. They are characterized by cytologic atypia and anaplasia. On CT scans, they often show central necrosis in a large tumoral mass with invasion and infiltration of adjacent structures in the mediastinum. Because of their aggressive nature, they are likely to produce hematogenous and lymphatic spread, both locally and distally.
Thymolipoma
CT scanning reveals a fatty mass interspersed with varying amounts of thymic soft tissue. The whole mass often consists of adipose tissue except for a thin rim of thymic tissue; this finding is consistent with soft tissue attenuation and mimics a pure lipoma of the mediastinum.
Thymic cyst
On CT, a thymic cyst appears homogeneous with water attenuation. The attenuation may vary, depending on the contents of the cyst. High attenuation may be present if the cyst contains proteinaceous fluid or blood from hemorrhage. A neoplasm with cystic degeneration may closely mimic a thymic cyst; associated soft tissue attenuation may help in their differentiation.
Thymic rebound
After initiation of chemotherapy, CT scans may reveal a decrease in the size of the thymus. Rebound in the form of overgrowth occurs a few months after completion of chemotherapy. Criteria for rebound include an increase in size of greater than 50%, as compared with the baseline volume.
Degree of Confidence
Chest CT performed with and without contrast enhancement is clearly superior to routine radiography. A small thymic tumor can easily be missed on a chest radiograph, whereas CT scans distinctly delineate the tumor. In addition, enhancement often helps in clearly differentiating the mass from the surrounding vascular structures. This is helpful in planning surgery.
On imaging studies, bilateral extension of the mass, absence of fat planes, and invasion of adjacent structures are indications of malignancy. MRI may show similar findings. The demonstration of encapsulation of the mass and the homogeneous enhancement of the capsule are indications of benign tumors; these structures are better imaged with CT. These 2 radiologic modalities often complement each other and can facilitate preoperative diagnosis and staging of the thymus neoplasms.
In cases of lymphoid follicular hyperplasia, the thymus may not always appear enlarged; this may be overlooked on CT.
False Positives/Negatives
CT is highly sensitive for thymomas. If the thymus appears grossly asymmetric or if it has a lobular configuration, the diagnosis of a thymoma should be strongly considered. However, with CT, thymomas cannot be distinguished from other thymic masses, unless fat is visible. Thymomas are usually homogeneous and show mild enhancement with the use of contrast. They tend to grow to one side of the mediastinum or the other.
When cystic degeneration, necrosis, and old hemorrhage are present, the affected areas are of low attenuation. Normal thymus that has undergone fatty infiltration and a pulmonary mass adjacent to the anterior mediastinum sometimes mimic a thymoma. In cases of encapsulated thymoma, CT often reveals a preserved fat plane completely surrounding the mass. Fibrous adhesions and inflammation may mimic invasion of the tumor.
On CT, invasive thymoma is often characterized by encasement of mediastinal structures and pericardial/pleural implants; in advanced cases, transdiaphragmatic spread is seen.
Magnetic Resonance Imaging
Findings
In some cases, MRI is superior to CT for visualizing the thymus and for differentiating it from the surrounding soft tissue. Mass lesions in the mediastinum have sufficiently different imaging characteristics to allow their distinction from normal structures and fat. MRI produces excellent cross-sectional images in the mediastinum without contrast enhancement; with CT, contrast material is often needed to properly identify a mass and to avoid mistaking blood vessels for a mass lesion.
Encasement or invasion of the vasculature, esophagus, and trachea and involvement of the pericardium, myocardium, and pleura are accurately detected with MRI.
In all adults, the thymus is visible on MRI. Distinction between higher intensity mediastinal fat and the relatively hypointense thymus is optimal on T1-weighted images because of the long T1 of the thymus. The progressive decrease in T1 with advancing age is commensurate with the fatty infiltration associated with advancing age. However, the T2 relaxation times of the thymus do not change with aging. The thymus usually appears thicker on MRI than on CT in patients older than 20 years.
On T1-weighted images, thymomas have medium signal intensity that is higher than that of skeletal muscle but that is lower than that of fat. On T2-weighted images, the signal intensity approaches or exceeds that of fat. The mass often appears mostly homogeneous; areas of cystic degeneration appear as areas of variable signal intensity on T1-weighted images. This appearance is the result of variations in protein content or of hemorrhage; however, on T2-weighted images, thymomas appear bright. The hypointense fibrous septa often gives the mass a lobulated appearance.
Thymic hyperplasia
Thymic hyperplasia and normal thymus share the same characteristics on MRI. In the case of thymolipoma, T1-weighted MRIs reveal high signal intensity, which represents fat; strands of intermediate intensity represent thymic tissue.
Thymic cyst
T1-weighted MRIs of thymic cysts reveal low signal intensity; T2-weighted images show high signal intensity consistent with the fluid component of the lesion. T1-weighted images naturally show high signal intensity if the cyst contains blood from hemorrhage or if it is rich in proteinaceous fluid.
In healthy children younger than 5 years, MRI shows the thymus to have a quadrilateral shape and biconvex lateral contours.16 In older children and adolescents, the thymus is triangular with straight, lateral margins. On T1-weighted images, the thymus appears homogeneous with a signal intensity slightly greater than that of muscle; on T2-weighted images, the signal intensity is close to that of fat.
Qualitative evaluation of gross thymic morphology (ie, size, shape, margins, and signal intensity) is usually sufficient for distinguishing normal thymus from abnormal thymus. The abnormal thymus generally is enlarged, multilobular, or inhomogeneous because of the presence of cystic degeneration, hemorrhage, septations, fibrosis, or calcification, as seen on pathologic sections. In patients with lymphoma, associated lymphadenopathy is helpful in distinguishing normal thymus from abnormal thymus.
On T1-weighted spin-echo images, thymic lipomas have areas of high signal intensity, because of their fat content; the signal intensity is similar to that of subcutaneous fat, with areas of intermediate signal intensity reflecting the presence of soft tissue. Although thymic lipomas can attain a large size, they invariably do not invade surrounding structures. However, they can cause mass effect on the surrounding structures because of their size.
MRI findings in thymic carcinoid tumors are nonspecific and are identical to those of thymoma.
On T1-weighted images, thymic carcinoma has higher signal intensity than muscle; on T2-weighted images, there is an increase in signal. Heterogeneous signals often reflect the presence of necrosis, cystic degeneration, or hemorrhage.
Teratomas have various appearances on MRI, depending on the composition of the tumor. They commonly contain fat, which is of high signal intensity on T1-weighted images. Cystic changes may also be present; such changes have low signal intensity on T1-weighted images, but they have increased signal intensity on T2-weighting.
MRI often shows the inhomogeneous nature of seminomas.
Lymphoma and residual tumor
The MRI signal characteristics of untreated lymphomas are different from those of treated lymphomas. Untreated lymphomatous tissue has high signal intensity, whereas a homogeneous, hypointense pattern is characteristic of inactive residual fibrotic masses in patients receiving successful therapy for lymphoma. A heterogeneous pattern with mixed hypointensity and hyperintensity is often seen in untreated nodular sclerosing Hodgkin disease. A heterogeneous pattern with mixed areas of low and high signal intensity on both T1- and T2-weighted images is seen in lesions containing mixed fat (high signal intensity) and fibrous tissue (low signal intensity). This pattern is seen after treatment of patients with sterilized tumors. Positron emission tomography (PET) CT may play a role in assessing for residual lymphoma.17,18
Degree of Confidence
With MRI of the thorax, motion artifacts may occur. Breathing motion and pulsation of the heart and great vessels can markedly degrade image quality. Hence, in general, fast imaging sequences and artifact reduction techniques must be used for MRI of the mediastinum and chest.
MRI provides information similar to that provided by CT in the evaluation of thymomas. MRI is particularly useful when an intravenous contrast agent cannot be administered for use with CT because the patient is allergic to the agent.
Nuclear Imaging
Findings
Thallium-201 (201 Tl) single photon emission CT (SPECT) is useful for the evaluation of thymic lesions associated with myasthenia gravis, including lymphoid follicular hyperplasia and thymoma.
On early images,201 Tl accumulation is more intense in thymomas than in the normal thymus and in lymphoid follicular hyperplasia. On delayed images, uptake is more intense in both thymoma and lymphoid follicular hyperplasia than in the normal thymus. Therefore,201 Tl SPECT can be used to differentiate normal thymus from lymphoid follicular hyperplasia and thymoma in patients with myasthenia gravis.
Degree of Confidence
201 Tl uptake is considered to reflect various factors, including cellular metabolic activity, regional blood flow, and the number of viable cells in the lesion; hence,201 Tl imaging is superior to CT in some respects.
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Further Reading
Keywords
thymus lesions, thymus tumors, thymic lesions, thymic tumors, thymic hyperplasia, thymoma, thymic cyst, carcinoma of the thymus, carcinoid tumor of the thymus and paraganglioma (thymic neuroendocrine tumors), nonseminomatous germ cell tumors, thymolipomas, thymic involvement in lymphoma and metastases, germ cell tumors (dermoid cyst, teratoma, seminoma)
