Thymus cancer is the uncontrolled growth of cells in the thymus, leading to the formation of tumors that can be thymomas or thymic carcinomas. The thymus is a lymphatic organ located in the anterior part of the mediastinum – the part of the chest that lies between the two lungs, in front of the heart, and behind the breastbone – and is an important part of the immune system.
The thymus is composed of different types of cells, which can develop into different types of cancer:
- Epithelial cells – provide structure and shape to the thymus. Thymomas and thymic carcinomas, which are the focus here, develop from these cells;
- Lymphocytes – compose most of the rest of the thymus. Whether in the thymus or in other parts of the immune system, it is lymphocytes that can develop into Hodgkin’s lymphoma and non-Hodgkin’s lymphoma; and
- Neuroendocrine cells or Kulchitsky cells – are less common cells that release certain types of hormones and can help develop carcinoid tumors.
Thymic cancer is rare but is the most common among tumors exclusive to the mediastinum. The causes and risk factors for thymic cancer are not known.
Types of thymic cancer
There are two types of tumors in the thymus:
Thymomas – tumors that grow slowly and rarely spread to other organs and parts of the body; and
Thymic carcinomas – tumors that are more aggressive than thymomas. They typically grow rapidly and often invade nearby organs at the time of diagnosis. Extrathoracic metastases can occur but are not common.
The World Health Organization (WHO) classifies thymomas using letters:
- Type A – oval or spindle-shaped epithelial cells. It is the rarest type of thymoma;
- Type AB – or mixed thymoma. It is similar to type A, but it has areas of mixed lymphocytes in the tumor;
- Type B1 – very similar to the normal structure of the thymus, but it has a large number of lymphocytes along with the normal-looking thymic cells;
- Type B2 – also has a large number of lymphocytes, but the thymic epithelial cells are larger and have irregular nuclei; and
- Type B3 – has few lymphocytes and is mainly composed of epithelial cells that resemble normal ones.
Type C is thymic carcinoma, the most aggressive form of thymus cancer. It has irregular-looking cells that may not resemble thymic cells at all.
Types AB and B2 are the most common.
Symptoms of thymus cancer
Many patients with thymus cancer are asymptomatic.
When symptoms are present, up to half of patients with thymomas develop myasthenia gravis, a disease caused by muscle weakness that initially presents with visual changes (weakness of the eye muscles), progressive weakness, and subsequent respiratory failure.
Other symptoms caused by thymic cancer include:
- Cough;
- Chest pain;
- Superior vena cava syndrome – causes difficulty breathing or shortness of breath, cough, chest pain, and swelling of the face, neck, trunk, and arms;
- Difficulty swallowing;
- Hoarseness;
- Loss of appetite; and
- Weight loss.
Diagnosis of thymic cancer
In the presence of typical symptoms of the disease, thymic tumors are detected through imaging tests, especially computed tomography (CT) scans and magnetic resonance imaging (MRI). These tests evaluate the presence of the tumor, its extent, whether any structure is compromised, and if it has spread.
Biopsy is performed to determine the type of tumor in the thymus.
The staging of thymic cancer has four stages:
- Stage I – the tumor is non-invasive (has not spread within the thymus capsule);
- Stage II – IIA: the tumor has grown into the thymus capsule; IIB: the tumor is in the thymus capsule, has reached nearby fatty tissues, and may have spread to the mediastinum;
- Stage III – the tumor has invaded neighboring tissues or organs in the lower neck or upper chest, including the pericardium (membrane around the heart), lungs, superior vena cava, and aorta; and
- Stage IV – IVA: the tumor has spread throughout the pleura and/or pericardium; IVB: the tumor has spread to distant organs such as bones, liver, and lungs.
Treatment for thymus cancer
The definition of treatment for thymus cancer depends on factors such as the type and staging of the tumor, whether it can be completely removed, and the patient’s overall health condition.
Thymus Cancer
When the tumor is well localized and restricted, the most common treatment is surgery with complementary radiation therapy in some cases. However, when the tumor is very large, chemotherapy may be used to shrink it before surgery. Chemotherapy is also employed in cases of more advanced thymus cancer.
Prevention of Thymus Cancer
Due to the lack of known causes and risk factors, it is not possible to prevent thymus cancer.
