Thymoma overview
IEditor-In-Chief: C. Michael Gibson, M.S., M.D. ; Associate Editor(s)-in-Chief:  Amr Marawan, M.D.  Ahmad Al Maradni, M.D. 

# Overview
- Thymoma is a benign thymic neoplasm located in the anterior mediastinum, behind the sternum and in front of the great vessels that involutes during puberty, it takes part in lymphocytes maturation throughout adulthood.
- The incidence of thymoma is approximately 0.13 per 100,000 individuals.
- Thymic neoplasm can be divided into two major groups: thymoma and thymic carcinomathymoma
- Thymoma is the most common tumor of the anterior mediastinum, consisting of any type of thymic epithelial cell as well as lymphocytes that are usually abundant and probably not neoplastic.
- Thymoma usually is benign, and frequently encapsulated uncommon tumor, best known for its association with the autoimmune disorder such as myasthenia gravis. Thymoma is found in 15% of patients with myasthenia gravis.
- Once diagnosed, thymomas may be removed surgically.  If left untreated thymoma may progress to invade the mediastinum and the surrounding structure.
- Depending on the stage of the tumor at the time of diagnosis, the prognosis may vary. However, the prognosis is generally regarded as good.
- Common complications of the thymoma include the pressure effect of the mass itself, autoimmune diseases, and rarely, malignancy.  Metastasis is extremely rare. In the rare case of a malignant tumor, chemotherapy may be used.
- Malignant lymphomas that involve the thymus, e.g., lymphosarcoma, Hodgkin's disease (termed "granulomatous thymoma" in the past), should not be regarded as thymoma.
- Thymomas associated with autoimmune disorders usually are benign. Malignant thymomas can metastasize, generally to pleura, kidney, bone, liver, or brain.

# Historical Perspective
- The thymic epithelial tumors staging was initially proposed by Bergh and his colleagues in 1978, modified by Wilkins and Castleman in 1979, and advanced by Masaoka et al. in 1981.

# Classification
- In 1999, a World Health Organization (WHO) Working group suggested a non-committal terminology (Masaoka classification), preserving the distinct categories of the histogenetic classification, but using letters and numbers to designate tumour entities.
- Recently, it has been very well accepted as it provides an easy comparison of clinical, pathological and immunological studies.

# Pathology
- On gross pathology, well circumscribed mass, that is locally invasive is a characteristic finding of thymoma.
- On microscopic histopathological analysis, round cells, with ample vacuolated cytoplasms, and fat droplets are characteristic findings of thymoma.

# Causes
- There are no established causes for thymoma.

# Differential Diagnosis
Thymoma must be differentiated from other thymic diseases such as
- thymic carcinoma
- Thymic cyst
- Thymic hyperplasia
- germ cell tumors.

# Epidemiology and Demographic
- The incidence of thymoma is approximately 0.13 per 100,000 individuals.
- Thymic neoplasms are the most common tumors located in the anterior mediastinum (20%).
- Incidence increases in middle age, and peaks in the seventh decade of life.
- Men and women are equally affected.

# Risk Factors
- There are no established risk factors for thymoma.

# Natural History, Complication and Prognosis
- If left untreated thymoma may progress to invade the mediastinum and the surrounding structure. Depending on the stage of the tumor at the time of diagnosis, the prognosis may vary.
- The prognosis is generally regarded as good.
- Common complications of the thymoma include the pressure effect of the mass itself, autoimmune diseases, and rarely, malignancy.

# Diagnosis

## History and symptoms
Symptoms of thymoma include
- muscle weakness
- cough
- wheezing
- dysphagia.
In addition to the symptoms of associated immune syndromes such as,
- anemia
- arthralgia
- skin rash.

## Physical examination
Patients with thymoma usually appear asymptomatic. Physical examination of patients with thymoma is may be remarkable for,
- Neck lump,
- Facial swelling
- wheezing.

## Staging
- Staging of thymic epithelial tumors was initially proposed by Bergh and his colleagues in 1978, modified by Wilkins and Castleman in 1979, and advanced by Masaoka et al. in 1981.
- Modified Masaoka staging grouped with TNM classification is the most widely adopted system for thymic epithelial tumors currently in use.

## Laboratory Findings
Laboratory findings associated with thymoma may include,
- Antibodies to the acetylcholine receptor,
- Abnormal electrolytes, renal, and liver function tests.

## X-Ray
- On chest x-ray, thymoma is characterized by oval to rounded, well demarcated, asymmetric, homogeneous mass of soft tissue density on one side of the midline.

## CT Scan
- Computed Tomography scan may be diagnostic of thymoma. The tumor is generally located inside the thymus, and can be calcified.
- Increased vascular enhancement can be indicative of malignancy, as can be pleural deposits.

## MRI
- On thoracic MRI, thymoma is characterized by increased heterogenous signal on T2WI.

## Ultrasound
- Ultrasound is used to guide fine needle aspiration or core needle biopsy in patients with thymoma.

## Other Imaging Studies
- PET scan may be used in the diagnosis of thymoma.

## Other Diagnostic Studies
Other diagnostic studies for Thymoma include
- CT scan guided core needle biopsy
- CT scan guided fine needle aspiration
- mediastinoscopy
- Videothoracoscopy.

# Treatment

## Medical Therapy
- Chemotherapy and radiotherapy are used as adjuvant or neoadjuvant therapies.
- Neoadjuvant therpy may be administered prior to surgery to make the tumor resectable.

## Surgery
- Surgery is the mainstay of treatment of thymoma.

## Primary Prevention
- There are no primary preventive measures available for thymoma.

## Secondary Prevention
- Complete surgical resection may help to prevent the recurrence of thymoma.