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What is thymus cancer?
Thymus cancers are uncommon cancers that start in the thymus. This small organ is located just behind the breast bone (sternum) in the front part of the mediastinum, the space in the chest between the lungs. The thymus sits just in front of and above the heart.
The thymus is divided into 2 halves, called lobes. It has an irregular shape and a surface that is made up of many small bumps called lobules. The thymus has 3 main layers:
The medulla is the innermost part of the thymus.
The cortex is the layer surrounding the medulla.
The capsule is the thin covering over the outside of the thymus.
The thymus reaches its maximum weight of about 1 ounce during puberty, then slowly decreases in size during adulthood as it is gradually replaced by fat tissue.
The thymus is an important part of the body's immune system. During fetal development and childhood, the thymus is involved in the production and maturation of T lymphocytes (also known as T cells), a type of white blood cell. T lymphocytes develop in the thymus and then travel to lymph nodes (bean-sized collections of immune system cells) throughout the body. There they help the immune system protect the body from viruses, fungus, and other types of infections.
The thymus has different types of cells, each of which can develop into different types of cancer:
Epithelial cells give the thymus its structure and shape. Thymomas and thymic carcinomas, which are the main focus of the rest of this document, develop from these cells.
Lymphocytes make up most of the rest of the thymus. Whether in the thymus or in other parts of the body, these immune system cells can develop into cancers called Hodgkin disease and non-Hodgkin lymphoma, which are described in other documents from the American Cancer Society.
Kulchitsky cells, or neuroendocrine cells, are much less common cells that normally release certain hormones. These cells can give rise to cancers called carcinoid tumors. This document does not discuss carcinoid tumors of the thymus. Much of the information in the American Cancer Society documents Lung Carcinoid Tumor and Gastrointestinal Carcinoid Tumors also applies to carcinoids of the thymus.
Doctors can tell the different thymic cancers apart by how they look under the microscope and by the results of other lab tests done on tissue samples.
Thymomas and thymic carcinomas
Thymomas and thymic carcinomas are tumors that start from thymic epithelial cells. Not all doctors agree about the best way to describe and classify these tumors. In the past, thymomas were sometimes divided into benign (non-cancerous) thymomas and malignant (cancerous) thymomas, based on whether they had grown beyond the thymus into other tissues or organs. The view now held by most doctors is that all thymomas are potentially cancerous, and the best way to predict how likely they are to come back after treatment is to describe whether they have grown into tissues beyond the thymus (and if so, how far). This is done by the surgeon who notes whether or not the tumor appears attached to nearby organs and by the pathologist who looks at samples from the margins (edges) of the tumor under the microscope. The system used to describe the stage (extent) of thymomas is discussed in the section "How is thymus cancer staged?"
WHO classification system for thymomas
Most doctors also classify thymomas by how they look under a microscope. This is called the histologic type. The system used for this classification, which was developed by the World Health Organization (WHO), assigns letters to the different types of thymomas.
Type A: The cells in these tumors are spindle-shaped or oval epithelial cells that appear to be fairly normal looking. This is the rarest type of thymoma, but it seems to have the best prognosis (outlook).
Type AB: This type, also known as a mixed thymoma, looks like type A except that there are also areas of lymphocytes mixed in the tumor.
Type B1: This type looks a lot like the normal structure of the thymus. It has a lot of lymphocytes along with normal-appearing thymus cells.
Type B2: This type also has a lot of lymphocytes, but the thymus epithelial cells are larger with abnormal nuclei (the DNA-containing part of the cell).
Type B3: This type has few lymphocytes and mostly consists of thymus epithelial cells that look pretty close to normal.
Type C: This is the most dangerous form and is also known as thymic carcinoma. It contains cells that have a very abnormal appearance under the microscope. The cells may no longer even look like thymus cells. These tumors have often grown into (invaded) nearby tissues and/or metastasized (spread to distant tissues and organs) at the time they are found. This type of thymoma has the worst prognosis (outlook).
Type AB and type B2 are the most common types of thymoma, and type A is the least common. As you go from A to C, the outlook for survival tends to get worse, with type A having the best outlook, and type C having the worst. Still, for most types of thymoma, the stage (extent of growth and spread) is a better predictor of a person's outcome. Staging of thymus cancer is discussed in the section "How is thymus cancer staged?"
What are the risk factors for thymus cancer?
A risk factor is anything that affects your chance of getting a disease such as cancer. Different cancers have different risk factors. For example, exposing the skin to strong sunlight is a risk factor for skin cancer. Smoking is a risk factor for lung cancer and many others.
But risk factors don't tell us everything. Having a known risk factor, or even several risk factors, does not mean that you will get the disease. And many people who get the disease may not have had any known risk factors.
No specific inherited, environmental, or lifestyle risk factors have been strongly linked to thymoma or thymic carcinoma. Some studies have suggested a possible link with exposure to radiation to the upper chest area, but this has not been confirmed. The only known risk factors are age and ethnicity.
Age: The risk of this type of cancer goes up with age. This type of cancer is rare in children and young adults, is seen more often in middle-aged adults, and is most common in those in their 70s.
Ethnicity: In the US, this cancer most common in Asians and Pacific Islanders and least common in Whites and Latinos. It is more common in African Americans than in Whites.
Do we know what causes thymus cancer?
Not much is known about why thymus cancers develop in some people but not in others. Researchers have found some DNA changes that occur more often in thymus cancer cells than in normal cells. (DNA is the chemical in each of our cells that makes up our genes — the instructions for how our cells function.) However, they are still not certain why these changes occur in some people, exactly how the changes occur, and how the changes might cause cancer to form.
Can thymus cancer be prevented?
Because we do not know what causes most thymic cancers, it is not yet possible to know how to prevent them.
Can thymus cancer be found early?
Thymus cancers are uncommon, and there are no widely recommended screening tests for them (screening is testing for cancer in people without any symptoms). Still, in some cases these cancers can be found early.
About 4 out of 10 people with thymomas have no symptoms when their tumor is found. In most of these cases the tumor is seen by chance on a test (like a chest x-ray or CT scan) that is done for some other reason.
Thymomas are often associated with symptoms that are not directly caused by the tumor mass itself. These are called paraneoplastic syndromes (tumor-related conditions). Some of these paraneoplastic syndromes, such as myasthenia gravis, red cell aplasia, and hypogammaglobulinemia, are described in the section “How is thymus cancer diagnosed?” These conditions can be very important in the early diagnosis of some thymomas because they may be present while the tumor is still at an early stage.
How is thymus cancer diagnosed?
If there is a reason to think you might have a tumor of the thymus, your doctor will ask you about symptoms and use one or more exams or tests to find out if the disease is really present. Certain signs and symptoms might suggest that a person may have a thymus tumor, but tests are needed to confirm the diagnosis.
Signs and symptoms of thymus cancers
Many thymic tumors are found on an x-ray done for some other reason, before the patient has symptoms. The rest are brought to the attention of a doctor after a person starts to have symptoms. These may be related to the tumor itself, or they may be part of a paraneoplastic syndrome.
Although these signs and symptoms might be caused by thymus tumors, they can also be caused by other conditions. Still, if you have any of these problems, it's important to see your doctor right away so the cause can be found and treated, if needed.
Symptoms caused by the tumor
The thymus is in the middle of the chest, near the airways and certain blood vessels. Tumors in the thymus can press on nearby structures, causing symptoms such as:
Shortness of breath
Cough (which may bring up bloody sputum)
Loss of appetite
The thymus is near the superior vena cava, the main blood vessel bringing blood from the head and upper body to the heart. Tumors that press on this vessel can cause symptoms of superior vena cava syndrome, which can include:
Swelling in the face, neck, and upper chest, sometimes with a bluish color
Swelling of the visible veins in this part of the body
Feeling dizzy or light-headed
These are conditions that are related to the cancer but that are not caused directly by the tumor mass. For example, people with thymomas may develop autoimmune diseases, where the immune system starts to attack the body itself. Part of the normal function of the thymus is to help keep the immune system in check, which may help explain why this happens.
Myasthenia gravis: About 30% to 65% of people with thymomas also have myasthenia gravis (MG). This is by far the most common autoimmune disease associated with thymomas. In this disease, the immune system forms antibodies that block the chemical signals that signal the muscles to move. This causes severe muscle weakness. People with MG tire easily. They may notice problems climbing stairs or walking long distances.
Although patients have decreased muscle strength throughout the body, symptoms caused by weakness of the muscles of the eyes, neck, and chest may be the most troublesome. Weakness of the eye muscles can cause blurred or double vision and drooping eyelids, while weak neck muscles can lead to problems with swallowing. Weakness of the chest muscles and diaphragm can cause problems breathing and shortness of breath.
Many people with thymomas have MG, but most people with MG don’t have thymomas. Many people with MG have other, noncancerous abnormalities of the thymus gland. Myasthenia gravis can be treated by removing the thymus (whether or not a thymoma is present) or with medicines that either strengthen the chemical signals to muscles or weaken the immune attack on the muscles.
Red cell aplasia: Red cell aplasia, in which the body's ability to make new red blood cells is severely reduced, occurs in about 5% of thymoma patients. Red blood cells carry oxygen from the lungs to other tissues of the body. Reduced red blood cell production causes anemia (low red blood cell counts). Symptoms of anemia can include weakness, dizziness, shortness of breath, and tiring easily. The usual treatment is to remove the thymus gland.
Hypogammaglobulinemia: Hypogammaglobulinemia is a disorder in which the body makes low amounts of infection-fighting antibodies (also known as gamma globulins). This leaves the person susceptible to infections. Hypogammaglobulinemia develops in about 5% to 10% of thymoma patients. About 10% of patients with hypogammaglobulinemia have a thymoma. Unlike myasthenia gravis and red cell aplasia, removing the thymus does not help correct this disease.
Other autoimmune diseases: Many other autoimmune diseases have also been linked to thymoma. However, they are much less common than myasthenia gravis, pure red cell aplasia, or hypogammaglobulinemia. Some examples include:
Systemic lupus erythematosus
Most people who have these autoimmune diseases do not have a thymoma.
Medical history and physical exam
If you have signs or symptoms that suggest you might have a thymus tumor, your doctor will want to take a complete medical history to check for symptoms. You will also be asked about your general health.
A physical exam provides information about possible signs of thymic cancer and other health problems. Patients with thymic cancer will sometimes have a fullness that the doctor can feel in the lower neck area.
Thymomas are often diagnosed by recognizing the signs and symptoms associated with myasthenia gravis, hypogammaglobulinemia, or red cell aplasia.
If symptoms and/or the results of the physical exam suggest a thymus tumor might be present, more involved tests will likely be done. These might include imaging tests, lab tests, and other procedures.
Imaging tests use x-rays, magnetic fields, or radioactive substances to create pictures of the inside of your body. Imaging tests may be done for a number of reasons, including to help find a suspicious area that might be cancerous, to learn how far cancer may have spread, and to help determine if treatment has been effective.
A chest x-ray may be the first imaging test a doctor orders if he or she suspects a problem in the middle of the chest. It may be able to show if there is a tumor in the chest. In some cases, a chest x-ray may find tumors in people before they cause any symptoms (when the person is having the x-ray done for another reason). However, some thymomas are small or are in certain places that may not show up on a chest x-ray. If your doctor is still suspicious or if an abnormality appears on the chest x-ray, a CT scan may be ordered.
Computed tomography (CT) scan
The computed tomography (CT) scan is an x-ray procedure that produces detailed cross-sectional images of your body. Instead of taking one picture, like a regular x-ray, a CT scanner takes many pictures as it rotates around you while you are lying on a narrow platform. A computer then combines these into images of slices of the part of your body that is being studied.
Before any pictures are taken, you may be asked to drink 1 to 2 pints of a liquid called oral contrast. This helps outline the intestine so that certain areas are not mistaken for tumors. You may also receive an IV (intravenous) line through which a different kind of contrast dye (IV contrast) is injected. This helps better outline structures in your body.
The injection can cause some flushing (redness and warm feeling that may last hours to days). A few people are allergic to the dye and get hives. Rarely, more serious reactions like trouble breathing and low blood pressure can occur. Medicine can be given to prevent and treat allergic reactions. Be sure to tell the doctor if you have ever had a reaction to any contrast material used for x-rays.
You need to lie still on a table while the scan is being done. During the test, the table moves in and out of the scanner, a ring-shaped machine that completely surrounds the table. You might feel a bit confined by the ring you have to lie in while the pictures are being taken.
CT scans can have several uses:
CT scans of the chest can spot very small tumors and help determine the exact location and extent of the tumors.
CT scans can be helpful in staging a cancer (determining the extent of its spread). For example, they can show whether the cancer has spread to nearby lymph nodes or to the liver, kidneys, brain, or other organs.
CT scans can also be used to guide a biopsy needle precisely into a suspected tumor or metastasis. For this procedure, called a CT-guided needle biopsy, the patient remains on the CT scanning table while a radiologist advances a biopsy needle through the skin and toward the location of the mass. CT scans are repeated until the needle is within the mass. A biopsy sample is then removed and looked at under a microscope.
During or after treatment, CT scans may be used to see whether tumors are shrinking or have recurred (come back) in other parts of the body.
Magnetic resonance imaging (MRI) scan
Like CT scans, MRI scans provide detailed images of soft tissues in the body. But MRI scans use radio waves and strong magnets instead of x-rays. The energy from the radio waves is absorbed and then released in a pattern formed by the type of body tissue and by certain diseases. A computer translates the pattern into very detailed images of parts of the body. A contrast material called gadolinium is often injected into a vein before the scan to better see details.
MRI scans may be a little more uncomfortable than CT scans. They take longer — often up to an hour. You may be placed inside a large cylindrical tube, which is confining and can upset people with a fear of enclosed spaces (claustrophobia). Special, more open MRI machines can help with this if needed. The MRI machine makes buzzing and clicking noises that you might find disturbing. Some places will provide earplugs to help block this out.
MRI of the chest may be done to look more closely at thymus tumors. They are most often used when the patient can’t have a CT scan for medical reasons (like problems with the IV contrast). MRI images are also particularly useful in looking for cancer that may have spread to the brain or spinal cord.
Positron emission tomography (PET) scan
For a PET scan, you receive an injection of a substance that contains a radioactive atom. This is usually glucose (a type of sugar), but other substances that are attracted to thymoma cells may also be used. The amount of radioactivity is very low. The cancer cells in the body absorb large amounts of the radioactive substance. A special camera can then be used to create a picture of areas of radioactivity in the body. The picture is not finely detailed like a CT or MRI scan, but it can provide helpful information about your whole body.
A PET scan can help give the doctor a better idea of whether an abnormal area seen on another imaging test is a tumor or not. If you have already been diagnosed with cancer, your doctor may use this test to see if the cancer has spread to lymph nodes or other parts of the body. A PET scan can also be useful if your doctor thinks the cancer may have spread but doesn't know where.
Certain machines are able to perform both a PET and CT scan at the same time (PET/CT scan). This allows the doctor to compare areas of higher radioactivity on the PET scan with the more detailed appearance of that area on the CT. Combined PET/CT is used more often than PET (alone) in looking at thymomas.
Blood tests : Blood tests can't be used to diagnose thymomas directly, but they may still be helpful in some situations. For example, tests may be done to look for certain antibodies in the blood of people who may have myasthenia gravis or other autoimmune disorders. Other blood tests may be done to make sure a mass in the middle of the chest isn't a germ cell tumor or part of the thyroid gland.
If a thymoma is diagnosed, blood cell counts and blood chemistry tests are done to get an idea of a person's overall health, especially if surgery is planned. Also, tests for myasthenia gravis (MG) will be done before any surgery. This is because MG is very common in patients with a thymoma, and, if left untreated, it can cause problems with anesthesia during surgery. People getting chemotherapy also have regular blood tests to make sure the drugs aren't having unwanted effects on the bone marrow, kidneys, or other organs.
Types of biopsy procedures : Although signs, symptoms, and imaging tests can suggest that a thymic tumor is likely to be present, doctors can't be certain of the diagnosis without looking at the tumor under a microscope.
For most cancers, removal of a small sample of the tumor (known as a biopsy) is needed to confirm whether a tumor is present and, if so, to determine its type. This is rarely done for thymomas because doctors are concerned that doing a biopsy could allow the tumor to spread. Because of this, doctors often remove the entire tumor rather than do a biopsy. If the doctor suspects a different type of tumor, a biopsy may be done before surgery. Most often, a needle biopsy is done.
Needle biopsy : Tumors in the chest are sometimes sampled by needle biopsy. A long, hollow needle is passed through the skin in the chest. Imaging tests such as CT scans are used to guide the needle into the tumor so that a small sample can be removed to be looked at under the microscope. This procedure is done without a surgical incision or overnight hospital stay.
A possible downside of this test is that it may not always provide enough of a sample to make an accurate diagnosis or allow the doctor to get a good sense of the extent of the tumor.
In most cases, if the doctor believes that the patient has thymoma (based on CT findings and lab tests, especially in a patient with a paraneoplastic syndrome) and it can be removed with surgery, the doctor may operate without any biopsy. This can both provide enough of a sample for a diagnosis and treat the tumor at the same time. The specimen is sent to the lab after surgery to confirm the diagnosis. See the section on “Surgery for thymus cancer" for more information.
How is thymus cancer staged?
Staging is the process of finding out how far a cancer has spread. Your treatment and prognosis (the outlook for chances of survival) depend, to a large extent, on the cancer's stage.
Masaoka staging system
There is no single staging system for thymomas that all doctors agree on, perhaps because these tumors are so uncommon. The system most often used to stage thymomas is called the Masaoka system, although other systems exist. Staging in the Masaoka system is based on:
The extent of disease as seen on imaging tests such as CT or MRI scans
Whether the surgeon finds the tumor hard to separate from nearby tissues (indicating the tumor is invasive)
Whether the doctor sees tumor cells beyond the thymus when looking at the tumor sample under the microscope
The Masaoka system separates thymomas into 4 main stages.
The thymoma is non-invasive. That is, it has not spread into the capsule (outer layer) of the thymus.
Stage II, which is divided into IIA and IIB
Stage IIA: The thymoma is growing into the capsule (the outer layer of tissue of the thymus).
Stage IIB: The tumor has grown through the capsule into the nearby fatty tissue, and may be stuck to the mediastinal pleura (the thin layer covering the space between the 2 lungs) or the pericardium (the tissue sac containing the heart).
The thymoma is growing into nearby tissues or organs of the lower neck or upper chest area, including the pericardium (the tissue sac containing the heart), the lungs, or the main blood vessels going into or exiting from the heart (the superior vena cava and aorta).
Stage IV, which is divided into IVA and IVB
Stage IVA: The thymoma has spread widely throughout the pleura (lining of the lungs and chest wall) and/or pericardium.
Stage IVB: The thymoma has spread to distant organs. The most common sites of spread are bone, the liver, and the lungs.
Resectable versus unresectable cancer
The Masaoka staging system divides thymomas into several groups that help give doctors an idea about a person's prognosis (outlook). But for treatment purposes, doctors often use a simpler system based on whether these cancers are likely to be resectable (where all visible tumor can be removed by surgery) or unresectable.
In general terms, almost all stage I and II thymomas, most stage III thymomas, and even some stage IV thymomas are potentially resectable, but there are exceptions. Resectability is based on whether the tumor appears to have grown into nearby tissues or spread to distant sites, as well as on whether or not a person is healthy enough to have surgery.
Surgery is typically part of the treatment plan whenever possible. In some cases, other forms of treatment such as radiation therapy or chemotherapy may be recommended as well.
Other prognostic factors
The prognosis (the outlook for chances of survival) after treatment of a thymoma depends to a large extent on its stage. But other features are important as well, such as its cellular classification (described in the section "What is thymus cancer?") and whether the surgeon is able to remove the entire tumor.
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