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| Detailed Guide: Cancer of Unknown Primary |
What Is Cancer of Unknown Primary? |
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Cancers often spread from their primary site (the part of the body in which the cancer originally
developed) to one or more metastatic sites (other parts of the body). Cancers are classified
according to their primary site, when that site is known, regardless of where in the body they
spread. For example, a lung cancer that spreads to the lymph nodes, adrenal glands, and the liver,
is still classified as lung cancer and not as a lymphoma (cancer of the lymph nodes), adrenal
cancer, or liver cancer.
About 5% of cancers first appear in one or more metastatic sites, and the primary site is not
known. These cancers are called cancer of unknown primary, or CUP. The primary site of some
of these cancers may eventually be found by additional tests. When this happens, they are no
longer considered a cancer of unknown primary, and are renamed according to the newly
discovered site of origin. For example, a patient with a metastasis to a lymph node on the side of
the neck may be diagnosed with cancer of unknown primary. The appearance of the cancer under
the microscope might suggest that the cancer started in the mouth, pharynx (throat), or larynx
(voicebox). During a thorough exam of this area, a small cancer of the pharynx might be found.
From then on, the patient is understood to have pharyngeal cancer rather than cancer of unknown
primary, and receives treatment that is appropriate for that cancer. Similarly, if a cancer is
determined to have started in the lung, breast, etc., it is no longer considered a cancer of
unknown primary.
In most cases of CUP, the source of the cancer is never discovered. Even when the primary site
is not known, the appearance of the cancer under the microscope, together with information
about which organs it has already affected may help doctors predict what kinds of treatment
might help the patient live longer or might relieve certain symptoms.
Cancer Types
In addition to being classified according to their primary site, all cancers are also named
according to their appearance under the microscope.
Cancers formed by flat cells that resemble cells normally found on the surface of the skin or the
lining of the mouth, throat, or vagina are called squamous cell cancers. Cancers that develop
from gland cells are called adenocarcinomas. Gland cells are normally found in many organs of
the body, including some that are not usually thought of as glands. Even though the lungs or
intestines are not usually classified as glands, they have tiny mucus-producing glands in their
linings, and adenocarcinomas often develop there. By combining information about a cancer's
primary site and its microscopic appearance, one cancer might be named a "squamous cell cancer
of the lung" and another might be called an "adenocarcinoma of the pancreas."
Another type of cancer, lymphoma, develops from cells of the immune system found in lymph
nodes and several other organs. Melanomas develop from cells that produce the skin's tan or
brown color. Sarcomas develop from connective tissue cells that usually are present in tendons,
ligaments, muscle, fat, and related tissues. This list is not intended to include all types of cancers,
but merely to provide a few examples.
When the cancer cells closely resemble normal cells, the cancer is said to be well differentiated.
When they have very little similarity to normal cells, the cancers are said to be poorly
differentiated. Cancers with intermediate appearance are called moderately differentiated.
Sometimes there is so little resemblance to any normal cell type that doctors cannot tell what
type of cell the cancer started from. These cancers are called poorly differentiated malignant
neoplasms.
In order to best estimate the prognosis (the outlook for chances of survival) and select the best
treatment for patients with cancers of unknown primary, doctors classify these cancers according
to the location of the metastases and according to their microscopic appearance. In general, the
more sites involved, the poorer the prognosis. Also, if the cancer is in the liver or adrenal gland,
the prognosis is usually quite poor. Among cancers that have spread to other organs, those with
the best outlook are lymphomas, breast and prostate cancers and carcinoid tumors. Carcinoid
tumors can arise from hormone-producing nerve-like cells that are found in many organs. As
noted earlier, if tests eventually show the exact cell type or origin of the cancer, it is renamed and
no longer considered to be cancer of unknown primary.
Accurate classification can often help doctors select the best treatment for people with cancer.
Some cancers respond very well to specific treatments. Therefore, it is very important to classify
the cancer. This is best done by looking at the cancer under a microscope and performing special
tests in the laboratory. Once the better prognosis cancers have been excluded by certain tests
however, it becomes less important to find the exact origin or type of a cancer. In many cases,
tests to find a primary site might be uncomfortable and might require surgery and a long period
of recovery in the hospital. If their results are not likely to improve survival or quality of life, the
tests should not be done.
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