Molecular assays for the diagnosis of minimal residual head-and-neck cancer: Methods, reliability, pitfalls, and solutions

The prognosis of cancer patients is determined by the radicalness of treatm ent: residual tumor cells will grow out and develop in manifest local recur rences, regional recurrences, and distant metastases. Classical diagnostic methods such as radiology and histopathology have limited sensitivities, an d only by molecular techniques can minimal residual disease be detected. In tissue samples containing the normal tissue counterpart of a tumor, only t umor-specific markers can be exploited, whereas in other samples, tissue-sp ecific markers can be used. At present, there are two main methodologies in use, one based on antigen-antibody interaction and the other based on ampl ified nucleic acids, The most commonly used nucleic acid markers are mutati ons or alterations in tumor DNA (tumor-specific markers) or differentially expressed mRNA (tissue-specific markers). Many reports and reviews have bee n published on the assessment of minimal residual disease by molecular mark ers, showing either positive or negative clinical correlations. One of the main reasons for these contradictory findings is the technical difficulty i n finding the small numbers of tumor cells in the large number of normal ce lls, which necessitates sensitivities of the assays up to 1 tumor cell in 2 x 10(7) normal cells, These assays often are complex, demand considerable experience, and usually are laborious. In this review, we will address a nu mber of the technical issues related to molecular assays for tumor cell det ection that make use of nucleic acids as markers. Many difficulties in data interpretation are at least in part because of technical details that migh t have been solved by the incorporation of one or more appropriate controls . We hope that this review clarifies a number of these issues and help clin icians and investigators interested in this field to understand and weigh t he contradictory findings in the published studies. This will help move the field forward and facilitate clinical implementation.