You are in: eMedicine Specialties > Radiology > CHEST Thymus, LesionsArticle Last Updated: Jul 7, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: Tarakad S Ramachandran, MBBS, FRCP(C), FACP, Professor of Neurology, Clinical Professor of Medicine, Clinical Professor of Family Medicine, Clinical Professor of Neurosurgery, State University of New York Upstate Medical University; Chair, Department of Neurology, Crouse Irving Memorial Hospital Tarakad S Ramachandran is a member of the following medical societies: American Academy of Neurology, American Academy of Pain Medicine, American College of Forensic Examiners, American College of International Physicians, American College of Managed Care Medicine, American College of Physicians, American Heart Association, American Stroke Association, Royal College of Physicians, Royal College of Physicians and Surgeons of Canada, Royal College of Surgeons of England, and Royal Society of Medicine Coauthor(s): Uma I Raghunathan, MD, Consulting Staff, Department of Neurology, Franklin Medical Center; Dharmesh Patel, MD, Staff Physician, Department of Neurology, State University of New York Upstate Medical University; Leslie Kohman, MD, Vice-Chair of Academic Affairs, Program Director, Thoracic Surgery Residency Program, Professor, Department of Surgery, State University of New York Upstate Medical University; Joyce A Strohl, MD, FACS, FACCP, Consulting Surgeon in Thoracic Surgery, Private Practice, Flint, MI; Amar Swarnkar, MD, Director, Interventional Neuroradiology, Neuroradiology Fellowship Program, Assistant Professor, Department of Radiology, State University of New York Upstate Medical University Editors: Kitt Shaffer, MD, PhD, Director of Undergraduate Medical Education, Associate Professor, Department of Radiology, Cambridge Health Alliance; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; W Richard Webb, MD, Chief of Thoracic Imaging, Professor, Department of Radiology, University of California at San Francisco; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Kavita Garg, MD, Professor, Department of Radiology, University of Colorado Health Sciences Center Author and Editor Disclosure Synonyms and related 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) INTRODUCTIONSection 2 of 10
  BackgroundThymic 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, though aberrant parathyroid or thyroid tissue masses are sometimes found, along with vascular and mesenchymal tissue masses. 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. 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. PathophysiologyThymic epithelial tumors are classified into 3 groups:
Thymomas are further subclassified into medullary, mixed, and cortical thymomas. Cortical thymoma often follows a locally aggressive clinical course. Thymic carcinomas often comprise diverse histologic subtypes. The intermediate form has been classified as atypical thymoma. Thymic carcinomas usually lack a well-defined capsule. Areas of necrosis and hemorrhage are frequent within the gritty, gray-white tumor. Cystic changes are prominent in carcinomas arising from a thymic cyst. The association between thymic tumors and leukemia has been recognized for over a century. In 1974, Kaplan et al confirmed lymphoblastic lymphoma, a cancer of thymus-derived lymphocytes.1 These tumors are further classified as poorly differentiated and undifferentiated. On histologic evaluation, large-cell lymphomas are typically sclerotic. They are morphologically classified as follicular center-cell lymphomas; they involve either large cleaved cells or large noncleaved cells. Residual thymic structures are commonly identified. Persistent thymic tissue may form epithelium-lined cysts. Non-Hodgkin lymphomas (NHLs) include a low-grade lymphoma of the thymus that histologically resembles the lymphomas in mucosa-associated lymphoid tissue (MALT). They have prominent germinal centers. Various T-cell lymphomas had been described on the basis of histologic differentiation. Mediastinal Hodgkin disease is thought to originate from reticulum cells of the thymus and should be distinguished from other forms of Hodgkin disease. Four types are described: nodular sclerosis, mixed cellularity, lymphocyte predominant, and lymphocyte depleted. Focal involvement of the thymus by histiocytosis X is described. Histiocytosis X is a lesion of the Langerhans cell. Lesions can be encapsulated or infiltrative. Extrathoracic lesions might occur. Tumors may occur in association with other neoplasms, including those of Hodgkin disease, NHL, leukemia, and carcinoma. Metastases to the thymus can occur from the lung or from extrathoracic sites, as with spread from primary tumors of the genitourinary tract, gastrointestinal tract, or breast. Common primary sites include the kidney, testis, colon, rectum, ovary, prostate, breast, bladder, and stomach. Melanomas and sarcomas uncommonly metastasize to this site. Metastases can also occur from an unknown primary tumor. Most teratomas arise in the anterior superior mediastinum. In only a few cases have they been reported to originate in the posterior mediastinum. Overall, 15-20% of mediastinal teratomas are classified as malignant. Seminomas are the most common primary malignant mediastinal germ cell tumor. They almost always arise in the anterior mediastinum and are often intimately associated with the thymus. Nonseminomatous germ cell tumors include endodermal sinus tumor (yolk-sac tumor), embryonal carcinoma, choriocarcinoma, teratocarcinoma, and mixed germ cell tumors. They behave like seminomas. Thymic carcinoids histologically resemble thymoma, but they are aggressive. They are often encapsulated and large (mean diameter, 11 cm). On microscopy, they resemble carcinoid tumors from other sites. Mediastinal paragangliomas are more common in the anterior compartment than in the posterior mediastinum. The anterior ones are called aortic body tumors; those in the posterior compartment are aorticosympathetic lesions, paravertebral lesions, or paragangliomas. Patients may have several paragangliomas. Tumors of neural origin are usually posterior, though in rare cases they occur in the anterior mediastinum. These tumors include neurofibroma, schwannoma, malignant schwannoma, ganglioneuroma, ganglioneuroblastoma, and neuroblastoma. Thymolipoma is a rare, benign tumor of the anterior mediastinum; it is characterized by adipose tissue with interspersed strands of thymus. Like thymomas, thymolipomas can be associated with myasthenia gravis. In addition, they are described in patients with aplastic anemia, hypogammaglobulinemia, Graves disease, Hodgkin disease, or chronic lymphocytic leukemia. Lipomas may occur in the anterior mediastinum and can be large, measuring up to 30 cm in diameter. They are composed of mature adult adipose tissue with occasional lymphocytes. No thymic tissue is identified.2, 3 FrequencyUnited StatesThe incidence of thymic masses, as well as the clinical presentation, vary in accordance with patients' ages. In adults, the thymic masses that occur with greatest frequency are thymomas, followed by neurogenic tumors, lymphomas, and germ cell tumors. Neurogenic tumors are the most common tumors in children and infants, followed by foregut cysts, germ cell tumors, lymphomas, lymphangiomas and angiomas, tumors of the thymus, and pericardial cysts.
Mortality/Morbidity
Sex
AgeThe incidence of thymic masses, as well as the clinical presentation, vary in accordance with patients' ages. See also Frequency and Sex.
AnatomyThe thymus is located in the anterior mediastinum. Mediastinal tumors are often among the most difficult lesions to examine. Many different types of lesions occur in the mediastinum. Biopsy specimens are often inadequate, small, and crushed (see Images 1-2). Although the thymus may appear to be a single structure, it consists of a right lobe and a left lobe; the lobes are situated close to each other in the anterior mediastinum. The left lobe is usually slightly larger than the right, and it is the main caudal extension of the organ. In children, it is usually prominent. It usually extends from the level of the fourth costal cartilages to the lower poles of the thyroid gland. The sternohyoid and sternothyroid muscles, the manubrium, and the upper part of the body of the sternum and their adjacent costal cartilages lie in front of thymus. Behind it are the pericardium; the arch of the aorta, with its 3 large branches; the left brachiocephalic vein; and the trachea. The thymus is the site of lymphocyte maturation for the immune system. The thymus is predominantly composed of epithelial cells and lymphocytes. Precursor cells migrate to the thymus and differentiate into lymphocytes. Most of these lymphocytes are destroyed; those that are not destroyed migrate to tissues to become T lymphocytes. Ectopic intrathyroidal thymomas are rare, and they can be challenging to identify. Most patients present with an apparent thyroid nodule that is identified on imaging studies or clinical examination. Results of fine-needle aspiration are often misleading or indeterminate. Proper diagnosis requires appropriate histopathologic assessment and staining; a high clinical index of suspicion leads to the correct diagnosis. The morphology of the thymus varies drastically with age, and its normal size, weight, and consistency vary widely. Knowledge of these variations is essential for proper interpretation of imaging studies. In the neonate and young infant, the thymus weighs about 20 g. By puberty, it reaches a maximum weight of about 30 g. During this time, thymic parenchyma consists of a dense population of lymphocytes, separated by thin, fibrous septa. After puberty, involution takes place over a period of 5-15 years. A gradual reduction in the lymphocyte population is associated with fatty infiltration of the atrophied thymic follicles. By the age of 40 years, fatty tissue dominates, and by approximately 60 years of age, the thymic remnant that is left is made up of fat. The inferior thyroid and internal thoracic arteries and corresponding veins vascularize the thymus. Efferent channels of lymphatics drain into parasternal, tracheobronchial, and brachiocephalic nodes. The thymus does not receive any afferent lymphatics.14 Clinical DetailsThymic hyperplasia Thymic hyperplasia is more commonly associated with myasthenia gravis than is thymoma. 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. Thymic hyperplasia may accompany other diseases, including Graves disease, Addison disease, and acromegaly. Thymoma Thymoma is a benign or low-grade epithelial neoplasm of the thymus. It is commonly located in the anterosuperior mediastinum; however, when the thymus is ectopic, thymomas can be found in the other regions of the mediastinum, as well as in the neck, pulmonary hilum, thyroid gland, lung, or pleura. Thymoma is a common anterior mediastinal tumor, though it is a rare thoracic tumor. It often occurs in adults in the fifth and sixth decades. In rare cases, it occurs in patients younger than 16 years. There is a slight female predominance. On histologic examination, thymomas contain a variable admixture of lymphocytes and epithelial cells. They are cytologically benign and are classified as invasive and noninvasive. Encapsulated thymomas are usually noninvasive or are locally invasive. Most thymomas measure 5-10 cm, though they vary in size. One third of thymomas adhere to or invade neighboring structures, including the great vessels, mediastinal pleura, and pericardium. In advanced cases, transdiaphragmatic spread occurs. Distant metastases are rare. About 10-15% of individuals with myasthenia gravis have thymoma, and about one third of patients with thymoma have myasthenia gravis. Approximately 40% of thymomas are found incidentally on routine radiographs. Symptoms, when present, include cough, chest pain, dyspnea, dysphagia, and/or superior vena cava syndrome; constitutional symptoms may include fever and weight loss. About 10% of patients have associated red cell aplasia, hypogammaglobulinemia, and collagen vascular diseases. Thoracotomy with histopathology is best for assessing the prognosis. Encapsulated tumors rarely recur after total excision; radiotherapy has a role in completely or incompletely resected thymomas. Although the natural history of ectopic cervical thymomas and the associated prognosis are not well established, most reports suggest a generally benign course, with the possibility of cure after surgical resection of fully encapsulated tumors. Boman first described ectopic thymoma in the cervical region in 1941. These rare tumors appear to be more common in female patients than in male patients (ratio, 7:1). They are typically found in the lower, anterolateral aspect of the neck. Thymomas are often adjacent to the lower thyroid pole or are located in the lower pole of the thyroid itself. When they are intrathyroidal, they are often misdiagnosed as Hashimoto thyroiditis or lymphoma of the thyroid on fine-needle aspiration. Intrathyroidal and other cervical thymomas must be differentiated from related tumors of thymic or branchial pouch origin, including hamartomatous thymomas, spindle epithelial tumors with thymus-like differentiation (SETTLE), and carcinoma showing thymus-like differentiation (CASTLE), which differ both histologically and clinically. Germ cell tumors Germ cell tumors are most commonly seen during the second to fourth decades. They are often found in the anterior mediastinum; they rarely occur in the posterior mediastinum. They often occur within or are adjacent to the thymus. Remnants of primitive germ cell layers, ectoderm, endoderm, or mesoderm give rise to these tumors. In the absence of gonadal primary and retroperitoneal nodes, they are considered malignant germ cell tumors of primary mediastinal origin. Mature teratomas are benign. Malignant germ cell tumors often include seminoma or nonseminoma. Malignant mediastinal germ cell tumors occur more commonly in males than females, whereas benign germ cell tumors occur equally in the 2 sexes. Mature teratoma Mature teratoma is the most common benign germ cell tumor of the mediastinum. It accounts for 60-75% of all cases. The tumors are cystic and multilocular, and they often contain gelatinous or sebaceous fluid. Seminoma Seminoma is the most common malignant germ cell tumor of the mediastinum. Thymic lymphoma About 60-70% of cases mediastinal Hodgkin disease involve the thymus through infiltration. Approximately 5-10% of patients with lymphoma present with primary mediastinal lesions; these lesions account for 10-20% of all primary mediastinal masses. Thymic neuroendocrine tumors Thymic carcinoids are the most common thymic neuroendocrine neoplasm. They are invasive, often infiltrating and invading adjacent structures. Spread often involves the lungs, lymph nodes, liver, and bone. They are called neuroendocrine because one third are associated with Cushing syndrome secondary to excess production of adrenocorticotropic hormone (ACTH). Approximately 15-20% involve multiple endocrine neoplasia. CT scanning is often useful in the diagnosis, though the findings may be nonspecific and may not be useful for differentiating these lesions from other thymic masses. Thymic carcinoma Thymic carcinoma belongs to a group of uncommon epithelial neoplasms. These tumors are characterized by cytologic atypia and anaplasia. CT scans 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. Thymolipoma Thymolipoma is one of the benign thymic neoplasms. This rare neoplasm is encapsulated. It is composed of mature adipose tissue, with islands of thymic tissue interspersed within the tumor. Thymolipomas gradually increase in size; at the time of diagnosis, they are usually large. Thymolipomas are more common in children and young adults than in older individuals. Thymic cysts may be found in their normal location in the anterior mediastinum, or they may be found anywhere along the their course of development; they may also be ectopic in foci. They are classified as congenital, inflammatory, degenerative, or neoplastic. Congenital thymic cyst develops as a unilocular cyst from a persistent thymopharyngeal duct. Inflammatory thymic cysts are characteristically multilocular. Various neoplasms, including thymomas, germ cell tumors, and lymphomas, are associated with cystic degeneration in the thymus itself. At times, cystic degeneration in the thymus may occur after the treatment of mediastinal neoplasms. Rebound thymic hyperplasia Rebound thymic hyperplasia is thought to be true hyperplasia of thymus; it is characterized by an increase in the size and weight of the thymus gland. Hypertrophy involves the whole gland, including the cortex and medulla. The thymus is known to become involuted in association with prolonged chronic illness, chemotherapy, Cushing syndrome, or general stress. Several months after recovery, the thymus may rebound with regrowth; such regrowth occurs more commonly in young adults and children than in adults.15 Preferred ExaminationCT is most valuable in the diagnosis of thymic lesions and anterior mediastinal masses. MRI may also be used. DIFFERENTIALSSection 3 of 10
Mediastinum, Germ Cell Tumors Other Problems To Be ConsideredAcquired dysplasia RADIOGRAPHSection 4 of 10
FindingsMany 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. CT SCANSection 5 of 10
FindingsFrom 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. 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 ConfidenceChest 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/NegativesCT 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. MRISection 6 of 10
FindingsIn 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 ConfidenceWith 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 MEDICINESection 7 of 10
FindingsThallium-201 (201Tl) 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, 201Tl 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, 201Tl SPECT can be used to differentiate normal thymus from lymphoid follicular hyperplasia and thymoma in patients with myasthenia gravis. Degree of Confidence201Tl uptake is considered to reflect various factors, including cellular metabolic activity, regional blood flow, and the number of viable cells in the lesion; hence, 201Tl imaging is superior to CT in some respects. INTERVENTIONSection 8 of 10
Conditions that may be treated with surgery include thymoma, other thymic tumors, myasthenia gravis with thymoma, and myasthenia gravis without thymoma. Thymomas, along with the entire thymus gland, should be removed by means of median sternotomy. If the thymoma involves the pericardium, great veins, and/or part of the lung, they can be removed.19 MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
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