Doctors can look at a sample of the tumour under a microscope to find out which type of cell the cancer started in.
Even when a primary tumour can't be found, tests can now give some good information about it. Your gender, the position of the secondary cancer and detailed laboratory information about the cancer cells give important clues.
Doctors usually group cancers by where they first started (the primary site), such as breast cancer or lung cancer. They also look at the type of cells. For example, cancers that start in epithelial cells are called carcinomas. Epithelial cells make up the tissue that covers the outside of the body (our skin) and lines the internal organs.
To find out what the type of cell is, your doctor takes a tissue sample (biopsy) and sends it to the laboratory to be looked at under a microscope. The doctor in the lab (pathologist) can look at the shape and structure of the cells.
They may also test the tissue sample using antibodies to look for tumour markers. Tumour markers are chemicals produced by some cancer cells. Some tumour markers are made by only one type of cancer, while others can be made by several different types of cancer.
The study of tumour markers is called immunohistochemistry (IHC). Tumour markers are often called IHC markers.
With cancer of unknown primary, knowing the type of cell gives your doctor a clue as to where the cancer is most likely to have started from, and can plan your treatment.
Types of CUP
Cancer of unknown primary (CUP) can be split into 5 main groups:
Adenocarcinoma starts in the gland cells. These cells line certain organs of the body, and release substances such as mucus or digestive juices. At least 60 out of 100 cancers of unknown primary (60%) are adenocarcinomas. These types of tumours have most commonly started in the lung, pancreas or bowel.
Squamous cell carcinoma
Squamous cells are flat cells found in the skin and in the lining of parts of the body such as the nose, throat, lungs, cervix and anus. The secondary tumours are usually noticed as swollen (enlarged) lymph nodes in the neck or groin.
Between 5 and 10 out of 100 cancers of unknown primary (5 to 10%) are squamous cell tumours.
About 5 in 100 cancers of unknown primary (5%) are neuroendocrine tumours (NETS). Carcinoid tumours are the most common of these. They develop mainly in the small bowel or other parts of the digestive system. But they can also occur in the lung, pancreas, kidney, ovary or testicle.
If this type of cancer can be identified, it can respond very well to treatment.
Poorly differentiated carcinoma
This type of cancer starts in the cells that line the body’s organs (epithelial cells). But the cells look so different from normal cells, the doctor can’t tell exactly what type of epithelial cell the cancer started in. Around 20 in 100 cancers of unknown primary (20%) are this type.
Poorly differentiated neoplasm
The cells of the tumour look so abnormal the doctor can’t tell what type of cell the cancer started in. About 5 out of 100 cancers of unknown primary (5%) are this type. But after further testing in the lab, some of these turn out to be carcinomas or other types of cancer such as lymphoma or melanoma.
Other cancer types
Occasionally cancers such as melanoma, sarcoma and germ cell tumours are diagnosed without finding the primary tumour. Doctors treat them following guidelines for these specific types of cancer, and not CUP.