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 lungs, in front of the heart, and behind the sternum bone – and it is an important part of the immune system.
The thymus is made up of different types of cells, which can develop different types of cancer:
- Epithelial cells – give structure and shape to the thymus. Thymomas and thymic carcinomas, which are the focus here, develop from these cells;
- Lymphocytes – make up most of the rest of the thymus. Whether in the thymus or in other parts of the immune system, it is the lymphocytes that can develop into Hodgkin’s lymphoma and non-Hodgkin’s lymphoma; and
- Neuroendocrine cells or Kulchitsky cells – these are less common cells that release certain types of hormones and can help carcinoid tumors develop.
Thymus cancer is rare, but is the most common among tumors unique to the mediastinum. Causes and risk factors for thymus cancer are not known.
Types of thymus cancer
There are two types of thymus tumors:
- Thymomas – indolent tumors, meaning that they are slow growing and rarely spread to other organs and parts of the body; and
- Thymic carcinomas – aggressive tumors, that is, tumors that grow quickly and spread to other organs and parts of the body.
The WHO (World Health Organization) classifies thymomas by letters:
- Type A – oval or spindle-shaped epithelial cells. This is the rarest type of thymoma;
- Type AB – or mixed thymoma. It is similar to type A, but has areas of mixed lymphocytes in the tumor;
- Type B1 – very similar to the normal thymus structure, but has a large amount of lymphocytes next to the normal looking cells of the thymus;
- Type B2 – also has a large amount of lymphocytes, but the thymus epithelial cells are larger and have irregular nuclei; and
- Type B3 – has few lymphocytes and consists mainly of epithelial cells that look like normal cells.
Type C is thymic carcinoma, the most dangerous form of thymic cancer. It has irregular looking cells, which may not even look like thymus cells.
Types AB and B2 are the most common.
Symptoms of Thymoma
Many patients with thymus cancer are asymptomatic.
When there are symptoms, about one-third of them have myasthenia gravis, a disease caused by muscle weakness that has as its initial symptoms visual changes ( eye muscles grow weak), progressive weakness, and a consequent respiratory failure.
Other symptoms caused by thymus cancer are:
- Chest pain;
- Superior vena cava syndrome – causes difficulty breathing or breathlessness, coughing, chest pain, and face, neck, trunk, and arm swelling;
- Difficulty swallowing
- Loss of appetite; and
- Weight loss.
Diagnosis of thymus cancer
Faced with typical symptoms of the disease, thymus cancer is detected by imaging tests, especially computed tomography and magnetic resonance imaging. They evaluate the existence of the tumor, its extent, whether any structures are involved, and whether it has spread.
A biopsy is performed to determine the type of tumor in the thymus.
The staging of thymus cancer has four stages:
- Stage I – non-invasive (the tumor has not spread inside the thymus capsule);
- Stage II – IIA: tumour has developed inside the thymus capsule; IIB: tumour is in the thymus capsule, has reached nearby fatty tissues and may have reached the mediastinum;
- Stage III – tumor has reached nearby tissues or organs in the lower neck or upper chest, including the pericardium (membrane surrounding the heart), lungs, superior vena cava, and aorta; and
- Stage IV – IVA: tumor has spread throughout the pleura and/or the pericardium; IVB: tumor has spread to distant organs, such as bones, liver, and lungs.
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 general health conditions of the patient.
When the tumor is well localized and restricted, the most common treatment is surgery with complementary radiotherapy in some cases. When the tumor is very large, chemotherapy can be used to shrink it before surgery. Chemotherapy is also used in more advanced cases of thymus cancer.
Because there are no known causes or risk factors, it is not possible to prevent thymus cancer.