The definition of a tumor is a mass of abnormally growing cells. Tumors can be either benign or malignant. Benign tumors have uncontrolled cell growth, but without any invasion into normal tissues and without any spread. A malignant tumor is called cancer when these tumor cells gain the propensity to invade tissues and spread locally as well as to distant parts of the body. In this sense, thyroid cancer occurs when cells of the thyroid gland grow uncontrollably to form tumors that can invade the tissues of the neck, spread to the surrounding lymph nodes, or to the bloodstream and then to other parts of the body. The most common types of cancers of the thyroid gland are derived from the cells responsible for thyroid hormone production. The general term for cancers that come from glandular tissue is adenocarcinoma. In the thyroid, the most common types of cancer are papillary adenocarcinoma of the thyroid (75-80%) and follicular adenocarcinoma of the thyroid (15%). Papillary thyroid cancer takes on a folded appearance under the microscope, which eases its diagnosis. Follicular thyroid cancer may closely resemble normal thyroid tissue, but as a malignancy, has a propensity to divide uncontrollably and invade and spread. The next most common type of cancer of the thyroid is called medullary thyroid cancer (5%), which is derived from the parafollicular cells of the thyroid. This is often associated with a family/genetic predisposition to develop certain types of cancers (see below). The other major type of thyroid cancer often described is called anaplastic thyroid cancer (2%). This cancer usually affects older people and is very aggressive. Other types of cancers, such as lymphomas (cancer of the lymph gland cells), sarcomas (cancer of soft tissues such as muscle or cartilage cells), or metastases (cancers from other sites that have spread to the thyroid gland) are also seen in the thyroid gland.
How is thyroid cancer diagnosed and staged?
Any thyroid nodule deserves attention. Once a thyroid nodule is noted, the next steps are all designed to determine if the nodule represents a benign growth or malignant tumor. The most common etiology behind a thyroid nodule is a small portion of benign functioning thyroid tissue, which must be differentiated from a thyroid cancer. Obviously a careful physical exam should be done by a physician, with attention to the examination of the neck to attempt to detect enlarge lymph nodes. Other laboratory tests are also usually done to determine the function of the thyroid gland. Tests that indicate an over-functioning gland point more toward the nodule being composed of benign functional tissue. A test to determine the etiology of a thyroid nodule is a nuclear medicine study with radioactive iodine. This test is efficacious because functioning thyroid tissue takes up iodine to produce normal thyroid hormones. Therefore, radioactive iodine will be preferentially taken up by normally functioning thyroid tissue and will show up on tests that are designed to detect radioactivity. Hence, a nodule composed of functioning thyroid tissue will appear "hot" in these nuclear medicine scans (i.e., expelling a large amount of radioactivity because of the concentration of active thyroid tissue). These "hot nodules" are almost always benign and often require no further work-up. Nodules that are "cold" (i.e., do not take up much iodine) are also often benign, though can be malignant in 15-20% of cases. Therefore, these especially deserve more attention and further work-up.
The first step in investigating a suspicious or cold nodule, and often the definitive step in diagnosis, is a fine needle aspiration (FNA), which involves placing a needle into the nodule and drawing up cells from it so that they can be analyzed. FNAs have a diagnostic accuracy of over 98%, though it is highly dependent on the physician's expertise in performing the test. After a diagnosis is made, further work-up is done to determine if there was spread of disease to the local lymph nodes and distant areas of the body including the lungs and bones. Hence, this includes a CT scan of the neck to evaluate lymph nodes and an MRI of the neck to evaluate muscle or tracheal involvement. Some also recommend a bone scan and chest x-ray, as thyroid cancer can (rarely) metastasize to the bones and lungs.
The staging of a cancer basically describes how much it is grown before the diagnosis has been made, documenting the extent of disease. Before the staging systems are introduced, first some background on how cancers grow and spread, and therefore advance in stage. Cancers cause problems because they spread and can disrupt the functioning of normal organs. One way thyroid cancers can spread is by local extension to invade through the normal structures in the throat and into adjacent structures in the neck. Although this uncommonly happens in this fairly indolent disease, this invasion can include the tracheal and esophageal extension, causing possible airway compromise and disruption of swallowing function. Thyroid cancer spreads most commonly by accessing the lymphatic system. The lymphatic circulation is a complete circulation system in the body (somewhat like the blood circulatory system) that drains into various lymph nodes. When cancer cells access this lymphatic circulation, they can travel to lymph nodes and start new sites of cancer. This is called lymphatic spread, and usually denotes a poorer prognosis. Thyroid cancer can commonly spread to the lymph nodes of the neck, though (especially with papillary thyroid cancer) this may not carry a worse outcome. The lymph nodes commonly involved in thyroid cancer are those found in the anterior portion of the neck, called the cervical or jugular lymph node chains. They can be found in front of the large muscles on either side of the neck that contract when the head is turned from side to side. Tumor cells that spread to the jugular lymph nodes can then spread to the "supraclavicular" lymph nodes (found behind the collarbone) and to other lymph nodes in the neck. Eventually, they can spread to lymph nodes in the chest, called the mediastinal lymph nodes.
Thyroid cancers can also spread through the bloodstream. Cancer cells gain access to distant organs via the bloodstream and the tumors that arise from cells traveling to other organs are called metastases. Cancers of the thyroid generally spread locally or to lymph nodes before spreading distantly through the bloodstream. Hence, the incidence of distant metastases is low, with less than 5% of papillary thyroid cancers showing distant spread and between 5 and 20% of follicular thyroid cancers exhibiting metastases. If spread through the bloodstream does occur, the lungs and bones are the most common organs involved.
The staging system used in thyroid cancer is designed to describe the extent of disease in both the thyroid itself and the neck (with spread to the lymph nodes). The staging system used to describe thyroid tumors is the "TNM system", as described by the American Joint Committee on Cancer. The TNM systems are used to describe many types of cancers. They have three components: T-describing the extent of the "primary" tumor (the tumor in the thyroid itself); N-describing the spread to the lymph nodes; M-describing the spread to other organs (i.e.-metastases).
The "T" stage is as follows:
- T1-tumor 1 cm or less within the thyroid gland
- T2-tumor sized 1-4 cm within the thyroid gland
- T3-tumor size greater than 4 cm within the thyroid gland
- T4-tumor of any size extending outside of the thyroid gland itself
The "N" stage is as follows:
- N0-no spread to lymph nodes
- N1-tumor spread to lymph nodes
- N1a-spread to lymph nodes on the same side of the neck as the primary tumor
- N1b-spread to lymph nodes bilaterally or to the opposite side of the primary tumor
The "M" stage is as follows:
- M0-no tumor spread to other organs
- M1-tumor spread to other organs
The overall stage is based on a combination of these T, N, and M parameters as well as age (to emphasize the fact that younger patients have a better prognosis) and type of thyroid cancer (to emphasize that papillary and follicular thyroid cancers have excellent prognoses while anaplastic thyroid cancers have poor prognoses).
- Papillary or Follicular Thyroid Cancer, age > 45 years
- Stage I-T1, N0, M0
- Stage II-T2-3, N0, M0
- Stage III-T4, N0, M0 or any N1, M0
- Stage IV-any M1
- Papillary or Follicular Thyroid Cancer, age 45 years
- Stage I-any M0
- Stage II-any M1
- Medullary Thyroid Cancer, any age
- Stage I-T1, N0, M0
- Stage II-T2, N0, M0
- Stage III-any N1, M0
- Stage IV-any M1
- Anaplastic Thyroid Cancer, any age
- ALL designated as Stage IV to denote the aggressiveness of anaplastic thyroid cancer
Though complicated, these staging systems help physicians determine the extent of the cancer, and therefore make treatment decisions regarding a patient's cancer. The stage of cancer, or extent of disease, is based on the information gathered through the various tests done (described above) as the diagnosis and work-up of the cancer is being performed.