What is carcinoma of unknown primary?

Carcinoma of unknown primary describes the rare occurrence when a cancer is discovered in one or more sites in the body, but the primary site is not clearly identified.

Sometimes additional testing may identify the primary site, in which case the cancer is no longer cancer of unknown primary and is renamed according to where the cancer started. Yet, for many patients, the source of their cancer is never determined.

Types/subtypes

Carcinomas begin in the epithelial cells and often are poorly differentiated. Two of the most common types of carcinoma of unknown primary are squamous cell carcinoma and adenocarcinoma.

Squamous cell carcinoma is formed by flat cells that look like those normally discovered on the surface of the skin or on the lining of the anus, cervix, esophagus, lungs, mouth, throat or vagina.

Adenocarcinomas develop from gland cells, which are found in various organs. The majority of these types of cancers are found in the colon, intestines and stomach. Also, approximately four of every 10 lung cancers are adenocarcinomas.

Other types of carcinoma of unknown primary include poorly differentiated and malignant neoplasm.

Risk factors, treatment

There are no specific risk factors for carcinoma of unknown primary, according to the American Cancer Society. However, individuals with a history of smoking, those with a poor diet, and those who are overweight or obese are more likely to be diagnosed with a carcinoma of unknown primary.

Exposure to ultraviolet radiation in sunlight is a risk factor for melanoma, which is another common source of carcinoma of unknown primary.

Early detection of carcinomas of unknown primary is unlikely. Treatments are most often geared toward prolonging a patient’s life. Knowing the primary site is necessary to guide treatment as the cancer that began in one place requires the same treatment when it spreads. However, treatment can still be successful.

Results of lab tests, the way the cancer appears under the microscope and the knowledge of which organs the cancer has already affected may help physicians predict the treatments that may be effective. Treatment options include radiation therapy, surgery, chemotherapy, hormone therapy and targeted therapy.

Looking ahead

Researchers are conducting studies designed to improve diagnosis and treatment of carcinoma of unknown primary.

New lab tests are expected to precisely classify carcinoma of unknown primary and better predict overall prognosis and treatment response. In addition, targeted therapies are being studied with the hope that these drugs will have a better effect than chemotherapy. Knowing the origin site may not be necessary once targeted therapies are developed for more of the specific molecular changes in cancer cells. Knowing the details of alterations in the DNA and RNA of cancer cells may become key to guide the selection of the treatments most likely to be effective for each individual patient.

More information is available at the following websites:

www.cancer.gov/types/unknown-primary/patient/unknown-primary-treatment-pdq

www.cancer.org/cancer/cancerofunknownprimary/

www.mdanderson.org/patient-and-cancer-information/cancer-information/cancer-types/cup/index.html

www.mayoclinic.org/diseases-conditions/carcinoma-unknown-primary/basics/definition/CON-20034348