[Tumor markers: essential diagnostic tools for radiologists]

A tumor marker is defined as a substance that is produced by the body in response to cancer or by the cancer tissue itself. Since discovery of the Bence-Jones protein in 1848 and AFP (alpha-fetoprotein), CEA (carcino-embryonic antigen), and SCC (squamous cell carcinoma antigen) in the 1960s and 1970s, a number of tumor markers have been developed for routine clinical laboratory tests. Some tumor markers are specific for one type of cancer, while others are positive for several cancer types. Furthermore, tumor markers are seen in non-cancerous conditions as well as in cancer. A positive result for one tumor marker does not necessarily indicate the presence of a malignant lesion. Because of insufficient sensitivity and specificity, most tumor markers cannot be used in screening for cancer in the early stage. When the presence of cancer is suspected, it must be confirmed using other diagnostic approaches such as radiological tests, pathological tests, or meticulous observation of disease progress. Tumor markers are useful in the follow-up of patients after treatment for malignant lesions, and they are usually superior to radiological tests in detecting recurrent lesions. When cancer is evaluated radiologically, knowledge of tumor markers is of great help to radiologists. Basic knowledge regarding tumor markers and pitfalls in their clinical usage are described.