Thymus is placed in the anterior portion of the mediastinum, due to this fact the diagnostics, access and management of its neoplasms is somewhat difficult, thus once there is a tumor occurred (thymomas, thymic carcinomas, thymic neuroendocrine tumors, thymic hyperplasia, thymic cysts, and thymolipomas), the contiguous organs are often involved in the process, making the conditions even more complicated.

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Needle biopsy

A physician’s hands are seen performing a needle biopsy to determine nature of lump either fluid-filled cyst or solid tumor.

Definition and Background of Thymoma

Thymus is a part of the immune system weighing only about 1 ounce it reaches its maturity only after puberty, afterward it gets shrunk as the issue is slowly being replaced by fat during adulthood; the gland responsible for the production and maturation of T lymphocytes that protects the body from various infections. Thymus consists of two lobes, which subdivides into smaller lobules.

thymusAlso, there are three layers in the thymus: medulla, cortex and capsule and three types of cell:

  • Epithelial cells may transform into thymomas and thymic carcinomas
  • Lymphocytes regardless their allocation they are the initial reason of Non-Hodgkin and Hodgkin lymphomas
  • Kulchitsky cells are responsible for the production of certain hormones and may be the triggering factor of carcinoid tumors

Etiology of Thymoma

Etiological factors of thymomas are not clearly known, however, some issues may be triggering factors of the thymic neoplasms, namely:

  • Autoimmune disorders (myasthenia gravis)
  • Side effect /complication after chemotherapy
  • Inflammatory processes of the anterior mediastinum (cysts)
  • Congenital pathology in the thymus development and embryogenesis
  • Exceeded growth of adipose tissue in the thymus may develop thymolipomas
  • Ectopic thymic tumors occur due to the abnormal formation of the thymus cells in other parts of the body: the neck, pericardium, pleura, and lung (ectopic hamartomatous thymoma (benign), spindle epithelial tumor with thymus-like differentiation (malignant), and carcinoma showing thymic-like differentiation (malignant)).

Epidemiology of Thymoma

Thymomas are rare indolently growing tumors, with an equal frequency of occurrence mid males and females, the most affected age group are 40- 50 years old people. The most aggressive thymomas behave in childhood.

Classification of Thymoma

Thymomas are divided into the two main groups benign (non-invasive) and malignant (invasive) neoplasms.

WHO classification system for thymomas (histological types):

Type A: Characterized by the appearance of spindle-shaped or oval epithelial cells normal looking (the rarest type of thymoma, has the best prognosis).

Type AB: Mixed thymoma, the most common type, similar to type A except that there are also areas of lymphocytes mixed in the tumor.

Type B1: This type has a normal structure of the thymus, contains a lot of lymphocytes as well as normal-appearing thymus cells.

Type B2: This type also has a lot of lymphocytes, but the thymus epithelial cells are larger. There are abnormal nuclei (the DNA-containing part of the cell).

Type B3: This type mostly consists of thymus epithelial cells close to normal and there are few lymphocytes.

Type C: Thymic carcinoma is the most aggressive tumor. Cells are with a very abnormal appearance, can hardly remind normal thymic cells. These tumors easily spread to distant tissues and organs when they are diagnosed.

Table: World Health Organization (WHO) Pathologic Classification and Associated Prognostic Categories

Type Histologic Description Disease-Free Survival at 10 years,* %
A Medullary thymoma 100
AB Mixed thymoma 100
B1 Predominantly cortical thymoma 83
B2 Cortical thymoma 83
B3 Well-differentiated thymic carcinoma 35
C Thymic carcinoma 28
*Series of 100 thymomas resected in Japan between 1973 and 2001 using WHO classification.


Presentation of Thymoma


Most of the thymomas have no symptoms (asymptomatic) in adults, the signs of the disease are sounder in childhood. Thymomas usually are found accidently, while radio diagnostics.

As the growth of the tumor continues, occupying a bigger space in the mediastinum, there may be:

In advanced forms there also may be:

  • Superior vena cava syndrome
  • Phrenic nerve paralysis
  • Hoarseness due to the involvement of recurrent laryngeal nerve
  • Persistent pleural effusions
  • Oncogenic set: weight loss, fever, fatigue, and night sweats

Physical Examination

The most evident symptoms are those ones that are associated with myasthenia gravis, hypogammaglobulinemia or red cell aplasia.

Diagnosis of Thymoma

Fine-Needle Aspiration (FNA) and Biopsy

  • Micrograph of adenoid cystic carcinoma. Pap stain.

    Image: “Micrograph of adenoid cystic carcinoma. Pap stain.” by Nephron. Licence: CC BY-SA 3.0

    Preoperative biopsy is compulsory while thymoma diagnostics

  • Limited anterior mediastinotomy (Chamberlain approach)
  • Thoracoscopic approach
  • Core biopsy in conjunction with FNA

Imaging Studies

  • Radiography detects the majority of thymomas which appears as the space occupying formations in the upper half of the chest overlapping the cardiac shadow close to the heart and large vessels.
  • CT determines smaller tumors , which were not detected in radiography
  • Positron emission tomography
By Tdvorak - Own work, GFDL, Link

Another axial slice of a CT scan of the chest showing a small thymoma anterior to the heart (marked with the red line). By TdvorakOwn work, GFDL, Link

Differential Diagnosis of Thymoma

Staging of Thymoma

Masaoka Staging System of Thymomas and Corresponding Therapy:

Stage Definition Treatment
I Encapsulated tumor with no gross or microscopic invasion Complete surgical excision
II Macroscopic invasion into the mediastinal fat or pleura or microscopic invasion into the capsule Complete surgical excision and postoperative radiotherapy to decrease the incidence of local recurrence
III Invasion of the pericardium, great vessels, or lung Complete surgical excision and postoperative radiotherapy to decrease the incidence of local recurrence
IVA Pleural or pericardial metastatic spread Surgical debulking, radiotherapy, and chemotherapy
IVB Pleural or pericardial metastatic spread Surgical debulking, radiotherapy, and chemotherapy


Management of Thymoma

The treatment of thymoma is mainly surgical, provided there is an appropriate access to the tumor, which makes it possible to remove it completely. If surgery is impossible then only chemotherapy, radiotherapy, and close monitoring are administered here. Corticosteroids contribute to regression of the neoplastic cells in the affected thymus that was proved in numerous tests and clinical cases.

The administration of a regimen of cisplatin/ vincristine/ doxorubicin/ cyclophosphamide for incompletely resected invasive thymomas or cases with unresectable disease (surgery is impossible due to the absence of surgical access to the affected area).

Long-Term Monitoring is a vital part of management

Relapse after primary diagnosed and cured thymoma may occur after 10-20 years. Hence, long-term monitoring should last throughout the patient’s life.

Review Questions

1. A 77 years patient complains about elevating after slight physical load breathlessness, dizziness, cough, and feeling of” heaviness” in the chest. Physical examination findings: there are not any lymph nodes enlarged in the neck, chest, and underarm. The skin is dry but with no cyanosis. ECG reveals normal age findings, CBC matches age specialties, electrolytes, and LDH are normal as well. Radiography indicated a smooth mass in the superior part of the mediastinum overlaying the heart shadow. Histological study (after FNA) indicates numerous adipose cells overlapping a number of thymic cells, which looks absolutely normal. What is the most probable diagnosis?

  1. Benign thymolipoma
  2. Medullary thymoma
  3. Mixed thymoma
  4. Predominantly cortical thymoma
  5. Cortical thymoma

2. An 81-years white man complains about cough, chest pain, hoarseness, high body temperature, night sweats, fatigue even without physical loads, weight loss. Physical examination reveals pallor; there are enlarged lymph nodes under arms, and infraclavicular region. CT of chest detected a formed mass in the mediastinum with an uneven surface, embracing heart on its front surface. FNA delivered some biomaterial where numerous epithelial cells were found, which hardly reminds normal thymic cells. CBC: elevated ESR. What is the most probable diagnosis?

  1. Benign thymoma.
  2. Thymic carcinoma
  3. NHL
  4. HL
  5. Tuberculosis

3. Biomaterial obtained from FNA of thymus revealed spindle-shaped or oval epithelial cells of usual appearance. What are the most probable diagnosis and disease-free survival at 10 years % for the assumed condition?

  1. Medullary thymoma, survival, 100 %.
  2. Predominantly cortical thymoma , 83 %.
  3. Cortical thymoma 83 %
  4. Well-differentiated thymic carcinoma 35 %
  5. Thymic carcinoma 28 %
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