THYROID
CANCER
Definition
Thyroid cancer is a disease in which the thyroid cells
become abnormal, grow uncontrollably, and form tumors. Thyroid
cancers are grouped into four types, based on how the cell appears
under the microscope. If left untreated, the cancer can spread to
other parts of the body.
Description
The thyroid is a butterfly-shaped gland, located at the
base of the throat. It has two lobes, the left and the right. The
thyroid gland makes hormones that regulate heart rate, blood
pressure, body temperature, and metabolism. The hormones produced
by the thyroid also affect the nervous system, muscles, and other
organs, and play an important role in regulating childhood growth
and development. The thyroid uses iodine, a mineral found in some
foods, to make several of its hormones.
Diseases of the thyroid gland occur frequently, affecting
millions of Americans. The most common diseases are an overactive
or an underactive thyroid gland. These conditions are called
hyperthyroidism (Grave's disease) or hypothyroidism. Sometimes,
lumps or masses may develop in the thyroid glands. Ninety-five
percent of these lumps or nodules are non-cancerous (benign), but
all thyroid lumps should be taken seriously.
According to the estimates of the American Cancer Society,
approximately 17,200 new cases of thyroid cancer will occur in the
United States in 1998. This disease accounts for 1% of all
cancers.
A woman's risk of developing thyroid cancer is three times
greater than a man's. Most people who develop thyroid cancer are
50 years of age or older, but the disease can affect teenagers and
young adults.
Thyroid cancers are grouped into four types, depending on
how the cells look under the microscope. The types are papillary,
follicular, medullary, and anaplastic thyroid cancers. The cancers
grow at different rates, so the aggressiveness of each cancer is
different.
Papillary cancer develops in the cells that produce thyroid
hormones containing iodine. It is a slow-growing cancer and can be
treated successfully. About 60-80% of all thyroid cancers are
papillary cancers.
Follicular cancers also develop in the cells that produce
iodine-containing hormones. Many of the follicular cancers have a
good cure rate, but if the cancer invades blood vessels or grows
into nearby structures in the neck, it may be difficult to
control. About 30-50% of thyroid cancers are follicular cancers.
Medullary cancers develop in the parafollicular cells (also
known as the C cells). These cells produce a hormone called
calcitonin, which does not contain iodine. These cancers are more
difficult to control because they have a tendency to spread to
other parts of the body. About 5-7% of all thyroid cancers are
medullary cancers. Approximately 7% of medullary cancers are
caused by the alteration (mutation) of a gene called the RET gene;
these cancers can be passed on in families.
Anaplastic cancer is the fastest growing of all thyroid
cancers. The cells rapidly spread to the different parts of the
body. About 2% of all thyroid cancers are anaplastic cancers.
Causes & symptoms
Although the exact cause of thyroid cancer has not yet been
determined, it has been observed that thyroid cancer affects women
three times as often as it affects men. The rate of thyroid cancer
is also higher in whites than in African Americans.
Exposure to radiation during childhood is a known risk
factor for thyroid cancer. In the 1950s and 1960s, radiation was
used to treat acne and to reduce swelling and infection of organs
in the neck, such as the tonsils, adenoids, and lymph nodes.
Recent studies have proved that people who received radiation to
the head and neck during their childhood have a higher than
average chance of developing thyroid cancer.
In areas of the world where people's diets are low in
iodine, papillary and follicular cancers occur more frequently. In
the United States, dietary iodine is plentiful because it is added
to table salt and other foods.
The most frequent symptom of thyroid cancer is a lump or
nodule that can be felt in the neck. Other symptoms are rare. The
lump usually is not painful, but some patients experience a tight
or full feeling in the neck and have some difficulty breathing or
swallowing. The lymph nodes may be swollen and the voice may
become hoarse because the tumor presses on the nerves leading to
the voice box.
Diagnosis
The doctor may use several tests to confirm a diagnosis of
thyroid cancer. The size and location of the lump has to be
identified and it has to be determined whether the lump is
non-cancerous (benign) or cancerous (malignant). Blood tests, such
as the thyroid stimulating hormone (TSH) test, may be ordered to
check how well the patient's thyroid is functioning.
A test known as the calcitonin test may be ordered if
medullary thyroid cancer is suspected. Calcitonin is a hormone
produced by the C cells (also called the parafollicular cells) of
the thyroid gland, in response to rising levels of calcium in the
blood. This hormone lowers calcium levels in blood serum by
slowing the rate at which the bones absorb it and increasing the
rate at which the kidneys discharge it. When the parafollicular
cells of the thyroid become cancerous, they produce too much
calcitonin. Because increased serum levels of the calcitonin
hormone are characteristic of medullary thyroid cancer, the
calcitonin test, which measures serum levels of calcitonin, can be
used to confirm a diagnosis of medullary thyroid cancer.
Imaging tests such, as a computed tomography scan (CT scan)
or an ultrasound scan, may be used to produce a picture of the
thyroid. In ultrasonography, high-frequency sound waves are
bounced off the thyroid. The pattern of echoes that is produced by
these waves is converted into an image by a computer. This test
can determine whether the lumps found in the thyroid are
fluid-filled cysts or solid malignant tumors.
A radioactive scan can be used to identify any abnormal
areas in the thyroid. The patient is given a very small amount of
radioactive iodine, which can either be taken by mouth or injected
into the thyroid. Since the thyroid is the only gland in the body
that absorbs iodine, the radioactive iodine accumulates there. A
x-ray image can then be taken or an instrument called a "scanner"
can be used to identify areas in the thyroid that do not absorb
iodine normally. These abnormal spots are called "cold spots" and
further tests are performed to check whether the cold spots are
benign or malignant tumors. If a significant amount of radioactive
iodine is concentrated in the nodule, then it is termed "hot" and
is usually benign.
The most accurate diagnostic tool for thyroid cancer is a
biopsy. In this process, a sample of thyroid tissue is withdrawn
and examined under a microscope. The tissue samples can be
obtained either by drawing out a sample of tissue through a needle
(needle biopsy) or by surgical removal of the nodule (surgical
biopsy). If thyroid cancer is diagnosed, further tests may be done
to learn about the stage of the disease and help the doctors plan
appropriate treatment.
Treatment
Treatment for thyroid cancer depends on the type of cancer
and its stage. Cancer staging considers the size of the tumor,
whether it has grown into surrounding lymph nodes, and whether it
has spread to distant parts of the body (metastasized). The
patient's age and general health status are also taken into
account. Four types of treatment are used: surgical removal,
radiation therapy, hormone therapy, and chemotherapy.
If the cancer has not spread to distant parts of the body,
surgical removal is the usual treatment. The surgeon may remove
the side or lobe of the thyroid where the cancer is found
(lobectomy) or all of it (total thyroidectomy). If the adjoining
lymph nodes are affected, they may also be removed during surgery.
Radiation therapy uses high-energy x rays to kill cancer
cells and shrink tumors. The radiation may come from a machine
outside the body (external beam radiation) or the patient may be
asked to swallow a drink containing radioactive iodine. Because
the thyroid cells take up iodine, the radioactive iodine collects
in any thyroid tissue remaining in the body and kills the cancer
cells.
Hormone therapy uses hormones to stop the cancer cells from
growing. When the thyroid gland is removed and levels of thyroid
hormones fall, the pituitary gland starts producing a hormone
called "thyroid stimulating hormone" (TSH). TSH stimulates the
thyroid cells to grow. This stimulation would also induce growth
of the cancerous thyroid cells. To prevent cancerous growth, the
natural hormones that are produced by the thyroid are taken in the
form of pills. Thus, their levels remain normal and inhibit the
pituitary from making TSH.
If the cancer has spread to other parts of the body and
surgery is not possible, the treatment is aimed at killing or
slowing the growth of cancer cells throughout the body.
Chemotherapy may be used, either in a pill or an injection through
a vein in the arm.
Prognosis
Like most cancers, cancer of the thyroid is best treated
when it is found early. More than 90% of patients who are treated
for papillary or follicular cancer will live for 15 years or
longer after having had thyroid cancer. Eighty percent of patients
with medullary thyroid cancer will live for at least 10 years
after surgery. The fourth type of thyroid cancer, anaplastic, is
usually fatal. Only 3-17% of patients with this cancer survive for
5 years.
Prevention
Because most people with thyroid cancer have no known risk
factor, it is not possible to completely prevent this disease.
However, inherited cases of medullary thyroid cancer can be
prevented. If a family member has had this disease, the rest of
the family can be tested.
The National Cancer Institute recommends that a doctor
examine anyone who has received radiation to the head and neck
during childhood at intervals of one or two years. The neck and
the thyroid should be carefully examined for any lumps or
enlargement of the nearby lymph nodes. Ultrasonography may be used
for people at risk for thyroid cancer.
