Tests for Cancer of Unknown Primary

Cancers of unknown primary (CUP) are usually found as a result of signs or symptoms a person is having. When the primary tumor cannot be located, specialized pathologic and molecular tests are used to help find where the cancer started, what kind of cancer cells they are, and to help guide treatment.

Medical history and physical exam

If you have any signs or symptoms that suggest you might have cancer, your doctor will want to take your medical history to check for symptoms and risk factors. This includes your family history. This will be followed by a physical exam that will pay special attention to any parts of the body where there are symptoms.

How is a cancer of unknown primary diagnosed?

If your symptoms and the results of your physical exam suggest cancer, the doctor may use the following types of tests to look for cancer, see what kind it is, find out where it is located, and where it might have started:

• Blood, plasma, and urine tests.
• Biopsies to remove samples of tissues or cells to look at them with a microscope or test them in the lab.
• Imaging tests such as x-rays, ultrasound, CT (computed tomography) or MRI (magnetic resonance imaging) scans
• Endoscopy exams to look at organs through a lighted tube placed into a body opening such as the mouth, nose, or anus

If these tests do not identify a primary site, a diagnosis of CUP is made.

The main goal in trying to determine the source of a CUP is to see if you have a cancer that may respond well to specific treatments. Some of the most important to identify include thyroid, breast, colon, and prostate cancers:

• In men, a prostate examination and a prostate-specific antigen (PSA) test may be helpful.
• In women, a pelvic examination and mammography should be done.
• In women whose physical exam suggests metastatic breast cancer, a breast MRI should be performed even if the mammogram results are normal.
• A colonoscopy may be done in patients with intra-abdominal metastases if the pathology results suggest gastrointestinal cancer.
• In men whose physical examination suggests metastatic prostate cancer, the pathologist might look for signs of prostate cancer in the biopsied tissue even if blood PSA levels are not elevated.
• Tests of the cancer cells for thyroglobulin can identify many thyroid cancers, which are often effectively treated with radioactive iodine injections.

Blood tests to diagnose cancers of unknown primary (CUP)

If signs and symptoms suggest you might have cancer, blood tests are done to examine the number and type of blood cells and to measure levels of certain blood chemicals. 

Complete blood count

The complete blood count (CBC) can show if you have a low blood count (red blood cells, white blood cells, or platelets). Numbers of different blood cell types that are lower than normal may suggest that a CUP has spread to bones and replaced much of the normal bone marrow, where new blood cells are made.

Anemia (lower than normal numbers of red blood cells) might also mean there’s esophageal, stomach, or intestinal bleeding caused by the cancer. This could point to those organs as the site of the cancer's origin.

Blood chemistry tests

Tests of chemical levels in the blood can show how well certain organs are functioning, and in some cases, they might give a clue as to where cancer may be .

For example, abnormal liver function test results in a person with CUP may suggest the cancer is in the liver. The cancer may have started in the liver or may have spread to the liver from another part of the body. Other blood tests can tell how well the kidneys are working and if cancer has spread to the bones.

Serum tumor markers

Some types of cancer release substances known as tumor markers into the bloodstream. There are many different tumor markers, but only a few of them are helpful in figuring out the origin of a cancer, such as:

• Prostate-specific antigen (PSA): A high PSA level in a man suggests that a CUP may have started in the prostate.

• Human chorionic gonadotropin (HCG): High levels of HCG suggest a germ cell tumor, a type of cancer that can begin in the testicles, ovaries, the mediastinum (area between the lungs), or the retroperitoneum (area behind the intestines).
• Alpha-fetoprotein (AFP): This substance is produced by some germ cell tumors as well as by some cancers that start in the liver.
• Chromogranin A (CgA): CgA levels can go up with neuroendocrine cancers.

Other tumor markers that may be helpful include:

• CA-125: A high CA-125 level suggests that the cancer may have started in an ovary, fallopian tube, or the peritoneum. Cancers from the breast, lung, or digestive tract can also involve the peritoneum, but usually involve other areas as well. Tests for CA-125 in the blood and tumor samples are positive in most ovarian and fallopian tube cancers. These tests can be used to help determine if the primary tumor is likely to be from there or some other organ.

• Occasionally, cancer starts in the peritoneum. These cancers, called primary peritoneal carcinoma, look, act, and are treated like ovarian cancer.
• CA 19-9: A high CA 19-9 level suggests that the cancer may have started in the pancreas or bile ducts.

There are many other tumor markers, but they are less useful in patients with a CUP because their levels go up with many different cancers. For example, carcinoembryonic antigen (CEA) can go up if an adenocarcinoma is anywhere in the body.  Adenocarcinomas of the colon, lung, ovaries, pancreas, stomach and many others can cause the CEA level to rise.

Biopsies to diagnose cancers of unknown primary (CUP)

When physical examinations, imaging tests, and/or blood tests suggest a cancer diagnosis, a biopsy is necessary to confirm it. Some of the suspicious area  is removed and looked at with a microscope. Other lab testing on the sample may be done as well.

Different types of biopsies may be done depending on where a suspected tumor is located.

• Core needle biopsy
• Surgical biopsy
• Endoscopic biopsy 

For more detailed information see Types of Biopsies Used to Look for Cancer.

Thoracentesis or paracentesis  

If you have large amounts of fluid inside your chest in the area around your lungs (known as a pleural effusion) or in your abdomen (known as ascites), samples of the fluid can be removed with a long, hollow needle. Ultrasound often is used to guide the needle. The fluid is then looked at under a microscope to see if it contains cancer cells and, if so, to determine the type of cancer present. Thoracentesis is the medical term for removing fluid from the chest cavity. Paracentesis refers to removing fluid from the abdomen. These procedures are usually done using local anesthesia (numbing medicine) while you are awake.

Bone marrow aspiration and biopsy

These tests may be done to see if cancer has spread to the bone marrow, the soft inner part of certain bones where new blood cells are made.

A bone marrow aspiration and biopsy are usually done at the same time. These procedures are usually done with local anesthesia (numbing medicine). In most cases, the samples are taken from the back of the pelvic (hip) bone.

• Bone marrow aspiration removes a sample of the liquid part of the bone marrow.
• Bone marrow biopsy removes a sample of the solid part.

A bone marrow biopsy is usually done just after the aspiration. A small piece of bone and marrow (about 1/16 inch in diameter and 1/2 inch long) is removed with a slightly larger needle that is twisted as it is pushed down into the bone. Samples from the bone marrow are sent to a pathology lab, where they are looked at and tested for cancer cells.

Lab tests on biopsy samples from cancers of unknown primary (CUP)

All biopsy samples are first looked at with a microscope by a pathologist, a doctor who has special training in laboratory diagnosis of cancers. How the cancer cells look will often provide clues to where the cancer started. If the diagnosis isn’t clear, then further testing might help.

Lab tests on the tumor cells can determine if they have estrogen receptors (ER) and/or progesterone receptors (PR).  Finding them may help confirm a diagnosis of breast cancer. The presence of these receptors is also important in planning treatment, as cancers containing these receptors are likely to respond to hormone therapy. Genetic testing for alterations in the HER2 and BRCA genes, as recommended in patients with breast cancer, may also be useful in confirming a breast cancer diagnosis and in planning treatment.

If a breast cancer diagnosis cannot be confirmed, tests to look for lung cancer may be done. Lung cancer is the most common cause of cancer spread to underarm lymph nodes in men and can also be the cause in women.

Molecular cancer classifier assay (MCCA)

These assays, or tests, help predict what type of tumor, or subtype of tumor, was biopsied. An MCCA detects the pattern of genes that are expressed or turned on in the cancer biopsy. It compares this pattern to a database of known cancer patterns from various specific sites. If there is a match using this method, doctors can accurately predict the tissue of origin in the large majority of patients with CUP. These tests are also known as gene expression profile assays.

Immunohistochemical (IHC) staining

For this test, a part of the biopsy sample is treated with man-made proteins called antibodies. These antibodies are designed to attach only to a specific substance found in certain cancer cells. If the patient’s cancer contains that substance, the antibody will attach to the cells. Chemicals are then added so that cells with antibodies attached to them change color. The doctor who looks at the sample under a microscope can see this color change. Doctors often need to use many different antibodies to try to determine what type of cancer is on the slides. Combining the results of the MCCA and IHC tests can be even more helpful in narrowing down where a CUP started.

Comprehensive Molecular Profiling (CMP)

CMP looks for a broad group of genes in a biopsy sample to find genetic changes. These include genes most associated with breast cancer (BRCA and HER2), lung cancer (ALK), and others such as EGFR, BRAF, TRK, RET, ROS1, and FGFR. CMP is usually done on tissue biopsies, but blood-based liquid biopsies (circulating tumor DNA or ctDNA) is also possible. When these molecular alterations are found, they can help guide treatment decisions and help choose targeted therapies that may effectively treat the cancer.  

Imaging tests for cancers of unknown primary (CUP)

Imaging tests use sound waves, x-rays, magnetic fields, or radioactive substances to create pictures of the inside of your body.

Imaging tests may be done for many reasons, including:

• To look more closely at an abnormal area that might be a cancer
• To learn how far cancer may have spread
• To try to see where a cancer has started
• To help determine if treatment has been effective

CT scans

CT scans of the chest, abdomen, and pelvis are the most common imaging tests used to evaluate a patient for a CUP. CT or MRI scans of the sinuses and neck area may be used to look for small cancers that may have already spread to lymph nodes in the neck. A PET scan (or combined PET/CT scan) may be done as well.

CT scans of the chest, abdomen, and pelvis are typically used to try to exclude other types of cancers (such as lung cancer). Women might have tests to see if the cancer started in the breasts or ovaries.

Ultrasound or CT of the neck may be used to look for thyroid cancer.

Breast imaging

If the patient is female, a mammogram is usually done also. A woman who has normal results on a breast examination, a mammogram, and a breast ultrasound, should have a breast MRI, since this test can sometimes detect small primary cancers.

Endoscopy for cancers of unknown primary (CUP)

For endoscopy, the doctor puts a flexible lighted tube (endoscope) with a tiny video camera on its end into the body.

Endoscopes are named for the part of the body they examine. For example, an endoscope that looks at the main airways in the lungs is called a bronchoscope and the procedure is called a bronchoscopy.

The base of the tongue, the throat, and the larynx are deep inside the neck and not easy to see. Indirect pharyngoscopy and laryngoscopy use small mirrors to look at these areas. A fiberoptic laryngoscope (a flexible, lighted, tube inserted through the mouth or nose) can also be used to look in those areas, as well as deeper in the throat.

If the cancer is likely to have started in the head and neck area, the mouth, throat, larynx, esophagus (tube that connects the mouth to the stomach), trachea (wind pipe), and bronchi (tubes leading from the trachea to the lungs) will be examined very thoroughly. This exam, called panendoscopy, is done in the operating room while you are under general anesthesia (asleep).

The endoscope used to look at the inside of the colon is called a colonoscope and the procedure is called a colonoscopy.

Other common types of endoscopy include:

• Esophagogastroduodenoscopy (EGD, also called upper endoscopy) to look at the esophagus (the tube that connects the throat to the stomach), the stomach, and the duodenum (the first part of the small intestine)
• Cystoscopy to look at the bladder

If something suspicious is seen during the exam, biopsy samples may be removed with special tools used through the endoscope. The samples will be looked at with a microscope to see if cancer cells are present.

Endoscopic ultrasound (EUS): This test is done with an ultrasound probe attached to an endoscope. It’s most often used to get pictures of the pancreas and esophagus. In the esophagus, it can be used to look more closely at any tumors present. If there are no esophagus tumors, the endoscope travels through the esophagus and the stomach, and into the first part of the small intestine. The probe can then be pointed toward the pancreas, next to the small intestine. The probe is on the tip of the endoscope, so it’s a very good way to look at the pancreas. It’s better than CT scans for spotting small tumors in the pancreas. If a tumor is seen, it can be biopsied during this procedure.

A form of endoscopic ultrasound also can be used to look more closely at tumors of the rectum. For this procedure, the endoscope is passed through the anus and into the rectum.

Endoscopic retrograde pancreatography (ERCP): For this procedure, the endoscope is passed down the patient’s throat, through the esophagus and stomach, and into the first part of the small intestine. Through the endoscope, the doctor can see where the common bile duct connects to the small intestine.

A small amount of dye is then injected through the tube into the common bile duct and x-rays are taken. This dye is a contrast material, which helps outline the bile duct and pancreatic duct. The x-ray images can show narrowing or blockage of these ducts that might be caused by pancreatic cancer. The doctor doing this test can also remove cells through the tube to look at with a microscope to see if they look like cancer.

More information about these tests can be found in Endoscopy.

Classifying cancers of unknown primary (CUP)

After initial lab tests, the pathologist uses the results to classify a cancer of unknown primary into 1 of the 5 main types:

• Squamous cell carcinoma
• Adenocarcinoma
• Poorly differentiated carcinoma
• Poorly differentiated malignant neoplasm
• Neuroendocrine carcinoma