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A tumor marker is clinically useful if its results serve to separate a large heterogeneous population into smaller populations with more precisely predictable outcomes. In theory, if this separation is both reliable and disparate, one can apply therapy more efficiently to the population by exposing those most likely to need and benefit from the therapy while ensuring that the other group avoids needless toxicities. In essence, the term tumor marker has come to describe a variety of molecules or processes that differ from the norm in either malignant cells, tissues, or fluids in patients with malignancies. Assessing these alterations from normal can be used to place patients into categories that are distinguished by different outcomes, either in the absence of specific therapy or after various treatments are applied.

Tumor markers can include changes at the genetic level (for example, mutations, deletions, or amplifications), at the transcriptional level (for example, over- or underexpression), at the translational or posttranslational level (for example, increased or decreased quantities of protein, or abnormal gly-cosylation of proteins), and/or at the functional level (for example, histologic description of cellular grade or presence of neovascularization). Each of these can be assessed by one or more assays, which can be performed using one or more methods with differing reagents. This enormous heterogeneity of approaches is the root of considerable confusion regarding the true value, in clinical terms, of a given tumor marker.

The molecular revolution is now well into its fourth decade. Yet, in spite of impressive advances in our understanding of the biology of human malignancy and in the technology of investigating molecular processes, the number of clinically useful products from these advances is disappointing. For example, in 1995, the American Society of Clinical Oncology (ASCO) convened a panel of experts to establish guidelines for the use of tumor markers in colon and breast carcinoma. Although the Expert Panel reviewed many putative markers (including both tissue-based and circulating markers), their ultimate recommendations were surprisingly sparse (Table 7.1).1-3

Why are the ASCO guidelines so conservative? In reviewing the available literature, the Panel recognized that the science of clinical tumor marker investigation has been haphazard and relatively chaotic. Too often, studies of tumor markers are more inclined to be "fishing expeditions" with the hope that something interesting will be detected with statistical significance, rather than being prospective, hypothesis-driven investigations. In light of this confusion, several authors of the Guidelines separately developed a proposal for a framework in which previously published tumor marker studies might be critically evaluated in an evidence-based manner.4 The rest of this chapter reviews the generic concepts and policies related to tumor marker evaluation. Specific marker evaluation for a given disease are reviewed in the relevant chapter pertaining to that malignancy.

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