2.1　Conception of Biomarker in Tumor

An ideal tumor biomarker would be expected to have high sensitivity and specificity for cancer and fulfill certain criteria.It would be produced and released primarily by transformed cells or by other tissues in response to the tumor, at levels proportional to the state of differentiation and the mass of tumor cells.A therapeutic response or alteration in the state of the neoplastic tissue would be reflected in a change in the production of the biomarker such that tumor ablation or recrudescence would result in an appropriate change in the level of the biomarker.Furthermore, elevated biomarker levels would be seen in patients with small amounts of tumor present, so that residual primary or recurrent tumors could be detected at times appreciably earlier than would be possible by conventional diagnostic procedures [1].

Most of the tumor biomarkers currently available for clinical use do not fulfill these criteria.The only constant abnormality expressed in a cancer cell is that of growth inconsistent with the physiologic needs of the tissue where the tumor originates.This heritable alteration in growth is not accompanied by other consistent expression of either a single biochemical factor or a group of molecules that could be used as an imprint to distinguish a neoplastic from a normal cell [1].(https://www.daowen.com)

The detection of a molecular marker to serve as an indicator specific for a tumor depends upon the constant production of that molecule by the tumor, but not at all, or not above a background level,by none tumor cells.However, the production of the molecule may vary not only between similar tumors from the same organ, but also from the same tumor, since there is cellular heterogeneity within the same tumor represented by differences in the rate of growth, the stage of differentiation.Therefore, the expression of biochemical features could potentially be considered as biomarkers for malignancy.The production of such biomarker molecules may also be increased from normal tissues in response to certain physiologic stimuli, thus further confounding the utility of a molecule as a specific tumor biomarker [1].In practice, therefore, a tumor biomarker can be defined as a biochemical indicator that is usually found in abnormal concentration in the presence of a neoplasm.Virtually no tumor biomarker,however, has been shown to be specific or sensitive enough to use in the detection of tumors in the general population.