There are four types of cancer of unknown primary (CUP). A pathologist can determine the CUP type by looking at cells from the tumor under a microscope.

Adenocarcinoma

Approximately 60 percent of tumors designated as cancer of unknown primary are adenocarcinoma. These tumors are made of the epithelial cells that form glands. Adenocarcinoma can grow in most organs in the body because glands are involved in secreting substances or carrying those secretions.

Glandular epithelial cells also form the lining of most internal organs. This can make it difficult to identify the primary tumor site. Common primary sites for adenocarcinoma include the lungs, pancreas, breasts, prostate, stomach, liver, and colon.

Poorly Differentiated Carcinoma

About 20 to 30 percent of all cancers of unknown primary contain poorly differentiated cancer cells. These do not resemble normal cells and are usually more aggressive than other cancers.

Specialized testing may indicate whether these tumors began in lymph cells, skin cells, neuroendocrine cells (which secrete hormones into the blood), or other specialized cells. However, many poorly differentiated cancer cells appear so different from normal cells that doctors cannot determine their original cell type.

Squamous Cell Carcinoma

Squamous cell carcinoma accounts for less than 10 percent of cancers of unknown primary. It is made up of the flat epithelial cells that are found on the surface of skin or in the lining of certain organs, such as the mouth and esophagus. Some lung cancers are squamous cell carcinoma.

Neuroendocrine Carcinoma

Some cancers of unknown primary contain neuroendocrine cells. This is a highly diverse group of cell types that contain or produce hormones. Neuroendocrine carcinoma can begin anywhere in the body, making it difficult to identify the primary site.

For more information about neuroendocrine tumors, visit Gastrointestinal Neuroendocrine Tumors and Pulmonary Neuroendocrine Tumors.