Version July 2025
INTRODUCTION —
Thymic tumors are rare neoplasms that arise in the anterior mediastinum (figure 1). The clinical presentation, diagnosis, and staging of the most common thymic tumors, thymoma and thymic carcinoma, are presented here.
The management of thymoma and thymic carcinoma, and other related topics are discussed separately.
●(See "Treatment of thymoma and thymic carcinoma".)
●(See "Pathology of mediastinal tumors", section on 'Thymoma'.)
●(See "Pathology of mediastinal tumors", section on 'Thymic carcinoma'.)
●(See "Approach to the adult patient with a mediastinal mass".)
●(See "Thymic neuroendocrine neoplasms".)
EPIDEMIOLOGY —
In adults, thymomas and thymic carcinomas are the most common neoplasms arising in the thymus, which is located in the anterior mediastinum (figure 1). Thymomas account for approximately 20 percent of mediastinal neoplasms (table 1). Most patients with thymoma are diagnosed between 40 and 60 years of age. The incidence is similar in males and females [1]. There are no known risk factors, and there is a strong association with myasthenia gravis and other paraneoplastic syndromes.
CLINICAL PRESENTATION —
Thymomas and thymic carcinomas typically present in the following ways:
●As an incidental finding identified on imaging in an asymptomatic patient (image 1)
●Due to local (thoracic) symptoms
●Due to symptoms from a paraneoplastic syndrome
Thymoma
Thoracic manifestations — Thymomas typically are localized to the thymic gland and surrounding organs. Thoracic symptoms due to an anterior mediastinal mass are related to the size of the tumor and its effects on adjacent organs (eg, cough, chest pain, shortness of breath, hoarseness, phrenic nerve palsy). Some patients present with superior vena cava syndrome. (See "Malignancy-related superior vena cava syndrome", section on 'Clinical features'.)
Pleural or pericardial effusions are the most common manifestation of more disseminated disease and may also cause thoracic symptoms.
Paraneoplastic disorders — The most common paraneoplastic syndrome associated with thymoma is myasthenia gravis. However, a wide range of other autoimmune paraneoplastic syndromes has been reported (table 2).
Paraneoplastic syndromes may antedate the presentation with thymoma, be diagnosed concurrently with the thymic tumor, or occur after treatment (with or without evidence of tumor recurrence).
Myasthenia gravis and several other important paraneoplastic syndromes associated with thymoma are briefly discussed in this section. More detailed discussions are presented separately.
●Myasthenia gravis – Myasthenia gravis is an autoimmune disorder caused by interference by autoantibodies with acetylcholine receptors of voluntary muscle at the neuromuscular junction. Common symptoms include diplopia, ptosis, dysphagia, weakness, and fatigue. (See "Clinical manifestations of myasthenia gravis" and "Pathogenesis of myasthenia gravis".)
Up to one-half of patients with thymoma have symptoms consistent with myasthenia gravis [1]. Myasthenia gravis is common with all types of thymoma but is rare in thymic carcinoma. Males and females are equally affected.
Patients with thymoma and myasthenia gravis usually present with less advanced disease than those without myasthenia gravis, possibly because neuromuscular symptoms may lead to an earlier diagnosis [2-4]. In patients with thymoma and myasthenia gravis, thymectomy usually results in an attenuation of the severity of myasthenia gravis, although some symptoms persist in most patients [5]. (See "Overview of the treatment of myasthenia gravis" and "Role of thymectomy in patients with myasthenia gravis".)
●Pure red cell aplasia – Pure red cell aplasia results from an autoimmune-mediated hypo-proliferation of erythrocyte precursors in the bone marrow. This paraneoplastic disorder occurs in 5 to 15 percent of patients with thymoma and is more common in older adult females. Pure red cell aplasia is usually seen with tumors that have spindle cell morphology [6,7]. (See "Acquired pure red cell aplasia in adults".)
Although older reports suggested that thymectomy resulted in normalization of the marrow in up to 40 percent of cases [8,9], subsequent observations suggest that remission following surgical excision alone is uncommon [10]. Nonetheless, surgical resection of the thymoma is usually undertaken if pure red cell aplasia is found.
The management of pure red cell aplasia is discussed separately. (See "Acquired pure red cell aplasia in adults", section on 'Management'.)
●Immunodeficiency – Hypogammaglobulinemia and pure white blood cell aplasia are present in fewer than 5 percent of patients with thymoma, most commonly in older adult females [6]. Conversely, up to 10 percent of patients with acquired hypogammaglobulinemia have an associated thymoma (Good syndrome), typically of spindle cell histology [9]. (See "Primary humoral immunodeficiencies: An overview" and "Immune neutropenia".)
Patients usually have recurrent infections, diarrhea, and lymphadenopathy. As an example, in a review of 51 cases of thymoma-associated immunodeficiency, 19 patients had recurrent sinopulmonary infection, 12 had candidiasis (including 1 with candidemia), 7 had bacteremia, 12 had cytomegalovirus (only 5 with clinical manifestations), 6 had diarrhea, and several had a variety of other infections (including 3 with pneumocystis pneumonia) [11]. Thymectomy does not reliably lead to the return of normal immunoglobulin levels. (See "Primary humoral immunodeficiencies: An overview" and "Immune neutropenia".)
●Thymoma-associated multiorgan autoimmunity – Several case reports have described a syndrome of thymoma-associated multiorgan autoimmunity that is similar to graft-versus-host disease [12,13]. Patients presented with variable combinations of morbilliform skin eruption, chronic diarrhea, and elevations in liver function tests. Histopathology of the skin or bowel mucosa is similar to that seen with graft-versus-host disease. (See "Clinical manifestations and diagnosis of chronic graft-versus-host disease".)
Thymic carcinoma — Thymic carcinomas are more aggressive than thymomas; evidence of invasion of mediastinal structures is present in most patients [14]. As with thymomas, most patients present with cough, chest pain, shortness of breath, hoarseness, phrenic nerve palsy, or superior vena cava syndrome. Extrathoracic metastases are seen in less than 7 percent of patients at presentation. Metastatic disease due to thymic carcinoma is found most commonly in the liver and bone but may virtually develop in any site, including the brain, kidney, extrathoracic lymph nodes, adrenals, and thyroid [15].
DIAGNOSTIC EVALUATION
History and physical examination — For patients with a mediastinal mass that is suspected to be thymoma or thymic carcinoma, the diagnostic evaluation includes a thorough history and physical examination, supplemented by laboratory, imaging studies, and tissue sampling. The approach to history and physical examination in the adult patient with a mediastinal mass is discussed separately. (See "Approach to the adult patient with a mediastinal mass", section on 'History and physical examination'.)
Imaging of the primary tumor
Chest CT — The initial evaluation of an anterior mediastinal mass suspected to be a thymoma or thymic carcinoma should include a contrast-enhanced computed tomography (CT) of the chest. Such imaging can [16,17]:
●Establish the presence of an anterior mediastinal mass.
●Distinguish between a thymic malignancy and other benign etiologies, such as a thymic cyst or thymic hyperplasia, that do not require surgery. Of note, chest magnetic resonance imaging (MRI) typically has better discrimination than chest CT in distinguishing between thymic malignancy and thymic cysts or hyperplasia. (See 'MRI chest' below.)
●Provides the initial information to determine the resectability of the thymic tumor. As an example, a contrast-enhanced chest CT can demonstrate whether a suspected thymoma is well-circumscribed (image 2) or if it infiltrates surrounding structures (image 3).
●Provide initial evidence to distinguish between thymoma and thymic carcinoma. Thymic carcinomas often contain necrotic, cystic, or calcified areas (image 4) [18]. The tumor contour is often irregular compared with thymoma, which tends to be smooth [19].
MRI chest — A gadolinium-enhanced magnetic resonance imaging (MRI) of the chest is not routinely used in the diagnostic evaluation of thymic tumors. However, it can be used in certain clinical scenarios [16,17]:
●MRI chest is an alternative to CT chest in patients who cannot receive CT contrast (eg, allergy or poor kidney function).
●When CT is equivocal, an MRI chest typically has better discrimination than CT chest in distinguishing between thymic malignancy and thymic cysts or hyperplasia [20,21].
●MRI chest can be used to evaluate cystic or necrotic components of a mass and enhancing septations within a cyst.
●Chemical shift imaging can detect microscopic or intravoxel fat, which can distinguish a thymic neoplasm from hyperplasia.
●In patients who are determined to be surgical candidates on CT, MRI chest can be obtained during preoperative planning to better assess resectability and invasion of vital structures.
Differences in MRI signal pattern and fluorodeoxyglucose (FDG) positron emission tomography (PET)-CT scan appearance can also aid in distinguishing between thymomas and thymic carcinomas [22,23]. PET-CT scans are often negative in more well-differentiated thymomas but are often FDG-avid in thymic carcinomas [24,25].
Further details on the imaging evaluation of patients with an anterior mediastinal mass suspected to be thymoma or thymic carcinoma are discussed separately. (See "Approach to the adult patient with a mediastinal mass", section on 'Imaging'.)
Laboratory testing — The differential diagnosis of an anterior mediastinal mass includes not only thymoma and thymic carcinoma, but also retrosternal thyroid, lymphoma, and mediastinal germ cell tumor. Thus, preoperative evaluation may include acetylcholine receptor antibody test, germ cell tumor markers (beta-human chorionic gonadotropin [hCG], alpha-fetoprotein [AFP], and lactate dehydrogenase [LDH]) particularly in young males, as well as a complete blood count and pulmonary function tests. Further details are discussed separately. (See "Approach to the adult patient with a mediastinal mass", section on 'Laboratory studies'.)
Tissue diagnosis — The diagnosis of a thymoma or thymic carcinoma is confirmed by obtaining a tumor tissue sample and performing histopathologic analysis. Prior to obtaining tissue, multidisciplinary input that includes evaluation by a surgeon is necessary. (See "Approach to the adult patient with a mediastinal mass", section on 'Tissue diagnosis' and "Approach to the adult patient with a mediastinal mass", section on 'Thymoma and thymic carcinoma'.)
●For patients with a suspected thymoma or thymic carcinoma amenable to complete resection (based on initial imaging studies), the diagnosis can be definitively established through surgical resection, which is also part of management. (See "Treatment of thymoma and thymic carcinoma", section on 'Resectable disease'.)
●For patients with a suspected thymoma or thymic carcinoma that is not considered amenable to complete resection, or those who are ineligible for surgery (eg, due to age or comorbidities), a tissue diagnosis can be established with a core needle biopsy or an open or thorascopic biopsy.
Histopathology — The World Health Organization system is widely used to classify thymic neoplasms based on their histologic appearance (table 3) [26]. Of note, histologic classification may be difficult, particularly when classification is based on a biopsy with a relatively limited amount of tissue. Furthermore, it may be impossible to determine whether a thymoma is invasive based upon a biopsy alone. The pathology of thymomas and thymic carcinoma is discussed in detail separately. (See "Pathology of mediastinal tumors", section on 'Thymoma' and "Pathology of mediastinal tumors", section on 'Thymic carcinoma'.)
Evaluation for metastatic disease — For most patients with a confirmed diagnosis of thymoma or thymic carcinoma, we obtain systemic imaging to assess for metastatic disease using a FDG PET-CT (skull base to midthigh). One exception where we do not routinely obtain PET-CT imaging is for patients with small, clearly resectable thymomas for whom there is no clinical suspicion for more advanced disease.
In patients with thymomas, pleural metastases are more common than systemic involvement of distant organs and indicate unresectable disease. Careful attention should be paid to any pleural findings identified on CT chest. Patients with clinically suspected pleural implants on imaging should subsequently undergo thorascopic evaluation for confirmation. (See "Approach to the adult patient with a mediastinal mass", section on 'Surgical approach'.)
STAGING
Staging system — Thymomas and thymic carcinomas are staged according to the ninth version of the tumor, node, metastasis (TNM) staging of the American Joint Committee on Cancer (AJCC) (table 4) [27,28]. The ninth version AJCC staging system was updated to include tumor size in the T1 category (with tumors ≤5 cm classified as T1a and tumors >5 cm classified as T1b); and to reclassify tumor invasion of the lung or phrenic nerve as T2 (instead of T3) disease. There are no changes to the regional lymph nodes (N) and distant metastases (M) categories, and all stage groups remain the same [28]. Thymic neuroendocrine tumors, which are also staged using this system, are discussed separately. (See "Thymic neuroendocrine neoplasms", section on 'Staging system'.)
Other staging systems have also been used to stage thymic carcinomas, including the Masaoka staging system, which has been used in many studies and continues to be widely used (table 5) [29-34]. In this topic, the AJCC staging system is used unless otherwise noted. However, most of the data supporting treatment decisions have been based on patients staged using the Masaoka system, which is indicated when relevant.
Pathologic considerations — Staging of thymic neoplasms, including both thymomas and thymic carcinoma, is based upon the extent of the primary tumor and the presence of invasion into adjacent structures and/or dissemination. In approximately two-thirds of thymomas and thymic carcinomas, preoperative chest CT can accurately predict the pathologic TNM stage [35].
The AJCC TNM staging system does not distinguish between encapsulated tumors and tumors that invade through the capsule into the surrounding adipose tissue (picture 1). The presence of transcapsular spread should be noted when it is identified. Microscopic examination of possible invasion of adjacent organs and structures including the pericardium, mediastinal pleura, phrenic nerves, lung, and large vessels, amongst others, should also always be mentioned. Pleural implants and metastases need to be microscopically confirmed.
Thymomas and thymic carcinomas may be adherent to adjacent structures without invasion. In such cases, the surgeon should designate the site of adhesion on the specimen so that the pathologist can take sections from that area. An inflammatory fibrous reaction can also lead to the false impression of tumor invasion. The resection specimen should be oriented by the surgeon to identify the exact margins of the resection, which should then be inked. Radiopaque clips should be placed by the surgeon in areas of positive margins (or suspected positive margins) for later identification for possible radiation therapy (RT). This ensures the correct sites are irradiated when positive margins exist and postoperative RT is administered. (See "Treatment of thymoma and thymic carcinoma", section on 'Postoperative radiation therapy'.)
Furthermore, lymph nodes need to be sampled from the resection specimen for microscopic evaluation, noting their location in the mediastinum (N1 [anterior [perithymic]] versus N2 [deep intrathoracic or cervical]). Further details on the pathology of mediastinal tumors are discussed separately. (See "Pathology of mediastinal tumors".)
SOCIETY GUIDELINE LINKS —
Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Thymomas and thymic carcinomas".)
SUMMARY AND RECOMMENDATIONS
●Epidemiology – In adults, thymomas and thymic carcinomas are the most common neoplasms arising in the thymus, which is located in the anterior mediastinum (figure 1). Thymomas account for approximately 20 percent of mediastinal neoplasms (table 1). Most patients with thymoma are diagnosed between 40 and 60 years of age. The incidence is similar in males and females. (See 'Epidemiology' above.)
●Clinical presentation
•Common symptoms – Thymomas and thymic carcinomas typically present as an incidental finding identified on imaging, due to local (thoracic) symptoms, and due to symptoms from a paraneoplastic syndrome. (See 'Clinical presentation' above.)
•Thymic carcinoma – Thymic carcinomas are more aggressive than thymomas; evidence of invasion of mediastinal structures is present in most patients. Extrathoracic metastases are seen in less than 7 percent of patients at presentation. Metastases are found most commonly in the liver and bone but may develop in virtually any site. (See 'Thymic carcinoma' above.)
●Diagnostic evaluation
•Imaging of the primary tumor
-CT chest – The initial evaluation of an anterior mediastinal mass suspected to be a thymoma or thymic carcinoma should include a contrast-enhanced CT of the chest. (See 'Imaging of the primary tumor' above and "Approach to the adult patient with a mediastinal mass", section on 'Imaging'.)
-MRI Chest – While gadolinium-enhanced MRI of the chest is not routinely used in the diagnostic evaluation of thymic tumors, it can be useful in certain clinical scenarios (eg, alternative chest imaging for patients who cannot receive CT contrast; to discriminate between thymic malignancies and thymic cysts when the initial CT chest is equivocal; during preoperative surgical planning). (See 'MRI chest' above.)
•Laboratory testing – Laboratory testing may include germ cell tumor markers (beta-human chorionic gonadotropin [hCG], alpha-fetoprotein [AFP], and lactate dehydrogenase [LDH]), acetylcholine receptor antibody testing, as well as a complete blood count and pulmonary function tests. (See 'Laboratory testing' above and "Approach to the adult patient with a mediastinal mass", section on 'Laboratory studies'.)
•Tissue diagnosis – The diagnosis of a thymoma or thymic carcinoma is confirmed by obtaining a tumor tissue sample and performing histopathologic analysis. Prior to obtaining tissue, multidisciplinary input that includes evaluation by a surgeon is necessary. (See 'Tissue diagnosis' above.)
-Tumors amenable to complete resection – For patients with a suspected thymoma or thymic carcinoma amenable to complete resection (based on initial imaging studies), the diagnosis can be definitively established through surgical resection, which is also part of management. (See "Treatment of thymoma and thymic carcinoma", section on 'Resectable disease'.)
-Tumors not amenable to complete resection – For patients with a suspected thymoma or thymic carcinoma that is not considered amenable to complete resection, or those who are ineligible for surgery (eg, due to age or comorbidities), a tissue diagnosis can be established with a core needle biopsy or an open biopsy.
-Pathology – The World Health Organization system is widely used to classify thymic neoplasms based on their histologic appearance (table 3) The pathology of thymomas and thymic carcinomas is discussed separately. (See "Pathology of mediastinal tumors", section on 'Thymoma' and "Pathology of mediastinal tumors", section on 'Thymic carcinoma'.)
•Evaluation for metastatic disease – For patients with a confirmed diagnosis of thymoma or thymic carcinoma, we obtain systemic imaging to assess for metastatic disease using a fluorodeoxyglucose (FDG) positron emission tomography (PET)-CT (skull base to midthigh). One exception where we do not routinely obtain PET-CT imaging is for patients with small, clearly resectable thymomas for whom there is no clinical suspicion for more advanced disease. (See 'Evaluation for metastatic disease' above.)
Patients with thymoma and clinically suspected pleural implants on imaging should subsequently undergo thorascopic evaluation to assess for pleural metastases. (See "Approach to the adult patient with a mediastinal mass", section on 'Surgical approach'.)
●Staging
•AJCC staging system – Thymomas and thymic carcinomas are staged according to the tumor, node, metastasis (TNM) staging of the American Joint Committee on Cancer (AJCC) (table 4). (See 'Staging system' above.)
•Masaoka staging system – The Masaoka staging system has been used in many studies and continues to be widely used (table 5).
ACKNOWLEDGMENT —
The UpToDate editorial staff acknowledges Andrea Bezjak, BMedSc, MDCM, MSc, FRCPC, Giuseppe Giaccone, MD, PhD, and James R Jett, MD, who contributed to earlier versions of this topic review.
