Cancer of unknown primary has metastasized (spread) from another part of the body. The place where it began, also called the primary site, is unknown. The exact number of cases of CUP is not known, but it is estimated to be about 2% to 5% of cancers diagnosed in the United States. It is becoming less common because of better methods and technology to diagnose cancer.
About four of every five cases that are first diagnosed as cancer of unknown primary eventually are confirmed to be specific types of cancer. CUP most often is found in the lymph nodes, liver, lung, peritoneum (lining of the bowel) or bone.
Cancer of unknown primary often is challenging because:
- It tends to be aggressive
- The exact origin of cancer is unknown, so it may be more challenging to choose the best treatment. It often has spread to many sites in the body before it is found.
Cancer of Unknown Primary Types
Although doctors are at first unable to determine where CUP began, they study the cancer to find out as much as possible about it. After looking at the cancer cells under a microscope, they usually decide the cancer belongs in one of the following categories:
Adenocarcinomas: About six of every 10 cancer of unknown primary cases are adenocarcinomas, meaning they began in gland cells. Most frequently, the primary sites are the lung, pancreas, breast, prostate, stomach, liver or colon.
Poorly differentiated carcinoma: Cancer cells are present, but they do not have enough detail for doctors to decide the type of cancer. Around 10% of these CUP cases are found to be lymphoma, melanoma or sarcoma.
Squamous cell cancer: The cells are flat, similar to cells on the skin or linings of some organs.
Poorly differentiated malignant neoplasm: Cells are definitely cancer, but they are so abnormal it cannot be determined what type of cell or part of the body they began in. Sometimes they are lymphomas, sarcomas or melanomas.
Neuroendocrine carcinoma: This type of CUP begins in the neuroendocrine system, making the cells like nerve cells and hormone-making cells. They are scattered in organs including the esophagus, stomach, pancreas, intestines and lungs.
CUP Risk Factors
Because the primary site of many cancer of unknown primary cases is never found, it is difficult to say what causes them. In the cases in which the primary site is determined, possible risk factors are based on that type of cancer.
- Smoking tobacco is a risk factor in cancers of the pancreas, lungs, kidney, throat, larynx and esophagus. More than half of people with CUP have used tobacco.
- Diet and being overweight are linked to cancers that begin in the stomach, colon or rectum.
- Too much sun exposure causes some melanomas.
However, it has not been proven that any of these factors make a person more likely to develop cancer of unknown primary.