Cancer is characterized by an overgrowth of abnormal malignant cells in a specific region of the body. The disease begins in a primary site but may spread (metastasize) to other parts of the body. In carcinoma of unknown primary site (CUPS), cancer originally appears in one or multiple metastatic sites but the point of origin cannot be determined. This is usually because the initial tumor is too small to be detected or the initial site is difficult to view directly or with standard imaging techniques.

Recent advances in diagnostic assessment have made it possible to determine the origin of the cancer in most individuals, and these cases are then no longer considered CUPS. According to the National Cancer Institute (NCI), CUPS accounts for approximately 2 to 5 percent of all cancers. There are three main types of CUPS:

• Squamous cell carcinoma
• Adenocarcinoma
• Poorly differentiated malignant neoplasm

Because the exact type of cancer is unknown, it is difficult to identify the cause or risk factors of CUPS. However, cancer develops when DNA (the genetic code that governs a cell's development) becomes damaged during a patient’s life.  Factors such as smoking, unprotected sun exposure and eating an unhealthy diet may contribute to CUPS. Signs and symptoms of CUPS can be numerous and varied, depending on its type and location. They often include firm, swollen, nontender lymph nodes, shortness of breath, bone pain and skin lumps, among others.

Because CUPS has, by definition, already spread, there are no screening tests or routine cancer examinations that can help identify the tumors before they spread. Autopsies of these patients often reveal that the cancer began in the lungs and pancreas.

Treatment of CUPS depends on the classification and the location(s) of the metastatic cancer. Methods of treating this disease may include radiation therapy, chemotherapy, surgery and others. Because of the metastatic nature of this cancer, approximately 70 to 80 percent of patients diagnosed with CUPS will die within a year.