Thymic Lesion Imaging
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Thymic tumors occupy the anterior mediastinum, which is immediately posterior to the sternum and the anterior surface of the pericardium and great vessels. Tumors of thymic, lymphatic, or germ cell origin most commonly occur in this compartment, although aberrant parathyroid or thyroid tissue masses are sometimes found, along with vascular and mesenchymal tissue masses. (See the images below.)
About one half of all thymic tumors are malignant in individuals aged 20-40 years, and one third are malignant in persons younger than 20 years and those older than 40 years. (A malignant thymic tumor is seen in the image below.)
Approximately two thirds of all mediastinal tumors and cysts are symptomatic in children, whereas only a third are symptomatic in adults. When all age groups are considered, nearly 55% of patients with benign mediastinal masses are asymptomatic at presentation, compared with only approximately 15% of those in whom masses are found to be malignant. Computed tomography (CT) scanning is most valuable in the diagnosis of thymic lesions and anterior mediastinal masses. Magnetic resonance imaging (MRI) may also be used. [1, 2, 3, 4, 5, 6, 7, 8]
Many thymomas may be visualized using routine chest radiographs in posteroanterior (PA) and lateral views. A thymoma frequently 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 the images below.)
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. Thymolipomas 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.
From birth to puberty, CT scan 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.
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. Changes in the thymus over time are demonstrated in the image below.
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.
CT scanning may show symmetrical, diffuse enlargement of the thymus. When the enlargement is asymmetrical, it may mimic a thymoma; thymoma should be considered in the differential diagnosis. [1]
About two thirds of patients with myasthenia gravis have thymic hyperplasia; 25-50% of these patients are likely to have a normal thymus on CT scanning. Histologic analysis of the thymic medulla reveals numerous lymphoid follicles with active germinal centers.
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. (See the image below.)
On CT scans, seminomas are usually large and homogeneous, with soft tissue attenuation. Areas of low attenuation often are present secondary to necrosis and hemorrhage.
Nonseminomatous tumors include embryonal carcinoma, endodermal sinus tumor, choriocarcinoma, and mixed germ cell tumor. On CT scans, these lesions are often large and heterogeneous, with large (>50%) areas of low attenuation. They may contain areas of calcification.
On CT scans, 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, although the clinical picture and the presence of other sites of lymphadenopathy are often helpful in the diagnosis. (See the image below.)
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, locally and distally.
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. (See the image below.)
On CT scans, 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.
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.
Chest CT scanning 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 scanning. MRI and CT scanning 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 scans.
CT scanning is highly sensitive for thymomas. If the thymus appears grossly asymmetrical or if it has a lobular configuration, the diagnosis of a thymoma should be strongly considered. However, thymomas cannot be distinguished from other thymic masses on CT scans 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. (Images of thymomas appear below.) [2]
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 scanning often reveals a preserved fat plane completely surrounding the mass. Fibrous adhesions and inflammation may mimic invasion of the tumor.
On CT scans, invasive thymoma is often characterized by encasement of mediastinal structures and pericardial/pleural implants; in advanced cases, transdiaphragmatic spread is seen.
In some cases, MRI is superior to CT scanning for visualizing the thymus and for differentiating it from the surrounding soft tissue.
In healthy children younger than 5 years, MRI shows the thymus to have a quadrilateral shape and biconvex lateral contours. [9] 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.
Mass lesions in the mediastinum have sufficiently different imaging characteristics to allow their distinction on MRI from normal structures and fat, and 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 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.
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.
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.
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 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.) [3, 4, 10, 5, 6, 11]
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 scanning 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.
Thallium-201 (201 Tl) single photon emission CT (SPECT) scanning is useful for the evaluation of thymic lesions associated with myasthenia gravis, including lymphoid follicular hyperplasia and thymoma. [12]
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 thymoma and in lymphoid follicular hyperplasia than in the normal thymus. Therefore,201 Tl SPECT scanning can be used to differentiate normal thymus from lymphoid follicular hyperplasia and thymoma in patients with myasthenia gravis.
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 in some respects superior to CT scanning.
Ustaalioglu BB, Seker M, Bilici A, et al. The role of PET-CT in the differential diagnosis of thymic mass after treatment of patients with lymphoma. Med Oncol. 2010 Feb 13. [Medline].
El-Bawab HY, Abouzied MM, Rafay MA, Hajjar WM, Saleh WM, Alkattan KM. Clinical use of combined positron emission tomography and computed tomography in thymoma recurrence. Interact Cardiovasc Thorac Surg. 2010 Oct. 11(4):395-9. [Medline]. [Full Text].
Fujishita T, Kishida M, Taki H, Shinoda C, Miyabayashi K, Fujishita M, et al. Detection of primary and metastatic lesions by [18F]fluoro-2-deoxy-D-glucose PET in a patient with thymic carcinoid. Respirology. 2007 Nov. 12(6):928-30. [Medline].
Koopmans KP, de Groot JW, Plukker JT, de Vries EG, Kema IP, Sluiter WJ, et al. 18F-dihydroxyphenylalanine PET in patients with biochemical evidence of medullary thyroid cancer: relation to tumor differentiation. J Nucl Med. 2008 Apr. 49(4):524-31. [Medline].
Treglia G, Lococo F, Petrone G, Stefanelli A, Carnassale G, Calcagni ML, et al. A rare case of primary thymic Hodgkin lymphoma in an elderly patient detected by 18F-FDG PET/CT. Clin Nucl Med. 2013 May. 38(5):e236-8. [Medline].
Jackson T, Viner M, Subramaniam R. FDG PET/CT of carcinoma showing thymus-like differentiation. Clin Nucl Med. 2012 Jul. 37(7):718-9. [Medline].
Goldstein AJ, Oliva I, Honarpisheh H, Rubinowitz A. A tour of the thymus: a review of thymic lesions with radiologic and pathologic correlation. Can Assoc Radiol J. 2015 Feb. 66 (1):5-15. [Medline].
Kim KH, Seo HS, Lee YH, Lee KY, Kim YS, Son GS, et al. Study of intrathyroid fat-containing lesions using CT imaging with literature review. Neuroradiology. 2013 Nov. 55 (11):1405-11. [Medline].
Siegel MJ, Glazer HS, Wiener JI, Molina PL. Normal and abnormal thymus in childhood: MR imaging. Radiology. 1989 Aug. 172(2):367-71. [Medline].
Kumar A, Regmi SK, Dutta R, et al. Characterization of thymic masses using (18)F-FDG PET-CT. Ann Nucl Med. 2009 Aug. 23(6):569-77. [Medline].
Eguchi T, Yoshida K, Hamanaka K, Shiina T, Koizumi T, Kawakami S, et al. Utility of 18F-fluorodeoxyglucose positron emission tomography for distinguishing between the histological types of early stage thymic epithelial tumours. Eur J Cardiothorac Surg. 2012 May. 41(5):1059-62. [Medline].
Higuchi T, Taki J, Kinuya S, et al. Thymic lesions in patients with myasthenia gravis: characterization with thallium 201 scintigraphy. Radiology. 2001 Oct. 221(1):201-6. [Medline].
Tarakad S Ramachandran, MBBS, MBA, MPH, FAAN, FACP, FAHA, FRCP, FRCPC, FRS, LRCP, MRCP, MRCS Professor Emeritus of Neurology and Psychiatry, Clinical Professor of Medicine, Clinical Professor of Family Medicine, Clinical Professor of Neurosurgery, State University of New York Upstate Medical University; Neuroscience Director, Department of Neurology, Crouse Irving Memorial Hospital
Tarakad S Ramachandran, MBBS, MBA, MPH, FAAN, FACP, FAHA, FRCP, FRCPC, FRS, LRCP, MRCP, MRCS is a member of the following medical societies: American College of International Physicians, American Heart Association, American Stroke Association, American Academy of Neurology, American Academy of Pain Medicine, American College of Forensic Examiners Institute, National Association of Managed Care Physicians, American College of Physicians, Royal College of Physicians, Royal College of Physicians and Surgeons of Canada, Royal College of Surgeons of England, Royal Society of Medicine
Disclosure: Nothing to disclose.
Uma I Raghunathan, MD Consulting Staff, Department of Neurology, Bay State Neurology
Uma I Raghunathan, MD is a member of the following medical societies: American Academy of Neurology
Disclosure: Nothing to disclose.
Dharmesh Patel, MD Associate Neurologist, Neuro Medical Care Associates
Disclosure: Nothing to disclose.
Leslie Kohman, MD Distinguished Service Professor, Department of Surgery, State University of New York Upstate Medical University; Medical Director, Upstate Cancer Center
Disclosure: Nothing to disclose.
Joyce A Strohl, MD, FACS, FACCP Consulting Surgeon in Thoracic Surgery, Private Practice
Joyce A Strohl, MD, FACS, FACCP is a member of the following medical societies: American College of Chest Physicians, American College of Surgeons, Society of Thoracic Surgeons
Disclosure: Nothing to disclose.
Amar Swarnkar, MD, FRCR, MRCPI Director, Interventional Neuroradiology, Director, Neuroradiology Fellowship Program, Assistant Professor, Department of Radiology, State University of New York Upstate Medical University
Amar Swarnkar, MD, FRCR, MRCPI is a member of the following medical societies: American College of Radiology, American Society of Neuroradiology
Disclosure: Nothing to disclose.
Bernard D Coombs, MB, ChB, PhD Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand
Disclosure: Nothing to disclose.
W Richard Webb, MD Professor, Department of Radiology, University of California, San Francisco, School of Medicine
Disclosure: Nothing to disclose.
Kavita Garg, MD Professor, Department of Radiology, University of Colorado School of Medicine
Kavita Garg, MD is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, Radiological Society of North America, Society of Thoracic Radiology
Disclosure: Nothing to disclose.
Kitt Shaffer, MD, PhD
Kitt Shaffer, MD, PhD is a member of the following medical societies: American Roentgen Ray Society
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