Reference :Robbins basic pathology 9th edition pg no 225 : Tumor Markers Biochemical assays for tumorassociated enzymes, hor mones, and other tumor markers in the blood cannot be utilized for de nitive diagnosis of cancer; however, they can be useful screening tests and in some instances have utility in quantitating the response to therapy or detecting disease recurrence. The application of these assays is con sidered with many of the speci c forms of neoplasia dis cussed in other chapters, so only a few examples suf ce here. PSA, used to screen for prostatic adenocarcinoma, may be one of the most frequently and successfully used tumor markers in clinical practice. Prostatic carcinoma can be suspected when elevated levels of PSA are found in the blood. However, PSA screening also highlights problems encountered with use of viually every tumor marker. Although PSA levels often are elevated in cancer, PSA levels also may be elevated in benign prostatic hyperplasia (Chapter 17). Fuhermore, there is no PSA level that ensures that a patient does not have prostate cancer. Thus, the PSA test suffers from both low sensitivity and low speci city. PSA assay is extremely valuable, however, for detecting residual disease or recurrence following treatment for prostate cancer. Other tumor markers occasionally used in clinical practice include carcinoembryonic antigen (CEA), which is elaborated by carcinomas of the colon, pancreas, stomach, and breast, and alpha fetoprotein, which is pro duced by hepatocellular carcinomas, yolk sac remnants in the gonads, and occasionally teratocarcinomas and embry onal cell carcinomas. Unfounately, like PSA, both of these markers can be produced in a variety of nonneoplastic conditions as well. Thus, CEA and alpha fetoprotein assays lack both speci city and sensitivity required for the early detection of cancers. As with PSA screening, they are still paicularly useful in the detection of recurrences after excision. With successful resection of the tumor, these markers disappear from the serum; their reappearance almost always signi es the beginning of the end