Cáncer de tiroides

The thyroid gland is located in the front of the neck and is attached to the lower part of the voice box (larynx) and to the upper part of the windpipe (trachea). It has two sides, or lobes, that are connected by a narrow neck. The thyroid gland produces thyroid hormones, which regulate metabolism, growth, and development and are essential for life.

DIAGNOSING THYROID CANCER

Thyroid cancer may be suspected if a small abnormal growth—called a nodule—is found to protrude from the thyroid gland. Since the vast majority of thyroid nodules are benign, diagnostic tests must be conducted to determine if the nodule is malignant or cancerous.

Diagnosing thyroid cancer may involve tests that generate an image of the thyroid, such as ultrasound or PET imaging. A sample of the cells is also typically evaluated under a microscope. The sample may be removed using a needle and syringe or may be removed during surgery to treat the nodule. If initial tests indicate that the nodule is cancerous, a surgery will be scheduled to remove as much of the cancer as possible and to determine the extent of the disease—also called the stage of disease—and whether it has spread outside the thyroid gland.

Tests used to diagnose thyroid cancer include the following:

Ultrasound: Ultrasound uses high frequency sound waves and their echoes to create a two-dimensional image that is projected on a screen. Ultrasound is a simple procedure that may allow doctors to determine if a thyroid nodule is cancerous or benign based on the appearance of the image that is produced. A limitation of ultrasound is that it does not produce a sample of the cells that can be evaluated under a microscope.

Fine needle aspiration: Fine needle aspiration is a technique that uses a needle and syringe to withdraw a sample of the cells from a thyroid nodule. The cells can then be evaluated under a microscope to determine if they are cancerous or benign. Since many thyroid nodules are benign, this technique provides a minimally invasive way to determine if surgery is necessary.

Positron emission tomography (PET): Unlike techniques that provide anatomical images, such as X-ray or ultrasound, PET scans show chemical and physiological changes related to metabolism.

Before a PET scan, a patient will receive an injection of a drug that has a biological element—called an isotope—attached to it. The isotope becomes visible when a small amount of radiation is passed through the body. The most active cells take up more of the drug, allowing the doctor to see which areas are more active—a possible sign of cancer.

The radiation from a PET scan is roughly equivalent to what is administered in two chest X-rays. After the scan is complete, the radiation does not stay in the body for very long.  PET scans are covered by Medicare for the diagnosis of thyroid cancer.

TYPES OF THYROID CANCER

Cancer may arise from different cells of the thyroid gland. By evaluating a sample of the cancer under a microscope, doctors can determine the type of thyroid cancer. There are four main types of thyroid cancer:

Papillary: Papillary tumors are the most common form of thyroid cancer, accounting for more than 70% of all cases. Papillary cancers are typically irregular or solid masses that arise from otherwise normal thyroid tissue. More than half of papillary cancers have spread to lymph nodes in the neck. However, papillary cancers rarely spread to distant locations in the body. Papillary cancers typically occur in younger patients (30-50 years) and are commonly associated with a prior exposure to radiation. Patients with papillary cancer are highly curable with currently available treatment techniques.

Follicular: Follicular cancers account for a smaller percentage of all thyroid cancers (approximately 15%) and rarely occur after radiation exposure. Follicular cancers are more aggressive; they tend to invade blood vessels rather than lymph nodes, and distant spread is therefore more common. Potential sites of distant spread include the lung, bone, brain, liver, bladder, and skin. Patients over 40 have more aggressive disease that is more difficult to treat. Nonetheless, most follicular cancers are very curable.

Medullary: There are two subtypes of medullary thyroid cancer: sporadic and familial. Sporadic almost always occurs on both sides of the thyroid gland. Familial tumors may be malignant or benign and may be associated with a variety of symptoms.

Approximately half of medullary thyroid cancers have spread to lymph nodes. Prognosis depends on the extent of disease at diagnosis—especially spread to lymph nodes—and the ability to completely remove the cancer with surgery.

Anaplastic: Anaplastic thyroid cancer is a rare disease that may also be called undifferentiated cancer. This type of thyroid cancer is very aggressive, grows rapidly, and commonly extends beyond the thyroid gland. It typically occurs in older patients and is characterized by extensive spread in the neck area and rapid progression. Patients typically die of their disease within months of diagnosis.

STAGES OF THYROID CANCER

Following a diagnosis of cancer, the most important step is to accurately determine the stage of cancer. Stage describes how far the cancer has spread. Identifying the stage of cancer is important because each stage of cancer may be treated differently.

Stage I-II: Stage I-II thyroid cancers are generally confined to the thyroid, but many include multiple sites of cancer within the thyroid. Thyroid cancer that has spread to nearby lymph nodes is still considered to be in stage I-II when the patient is younger than 45 years of age as the presence of cancer in the lymph nodes does not worsen the prognosis for these younger patients.

Early stage thyroid cancer is very treatable and many patients are cured with surgery alone.

Stage III: Stage III thyroid cancer is greater than 4 cm in diameter and is limited to the thyroid or may have minimal spread outside the thyroid. Lymph nodes near the trachea may be affected. Stage III thyroid cancer that has spread to adjacent cervical (neck) tissue or nearby blood vessels has a worse prognosis than cancer confined to the thyroid. However, lymph node metastases do not worsen the prognosis for patients younger than 45 years.  Stage III thyroid cancer is also referred to as locally advanced disease.

Stage IV: Stage IV thyroid cancer has spread beyond the thyroid to the soft tissues of the neck, lymph nodes in the neck, or distant locations in the body. The lungs and bone are the most frequent sites of distant spread. Papillary carcinoma more frequently spreads to regional lymph nodes than to distant sites. Follicular carcinoma is more likely to invade blood vessels and spread to distant locations.

Recurrent: Thyroid cancer that has recurred after treatment or progressed with treatment is called recurrent disease.

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