Why Cytology for Molecular Testing? Pros and Cons

Abstract

In this era of precision medicine, molecular testing of tumor specimens has become indispensible in order to seek opportunities for optimized and personalized treatment in every cancer patient. Since most pivotal trials leading to the approval of targeted drugs rely on tissue specimens for predictive marker testing, biomarker testing on tissue biopsy specimens has long been considered the gold standard. In the real diagnostic world, however, malignant tumors are often diagnosed by cytology alone. This necessitates translating protocols for biomarker testing for the use in cytology, which comes with advantages and challenges. There is a general consensus that the quality of DNA obtained from ethanol-based cytological specimens is excellent allowing for highly accurate mutation analysis. Enrichment of tumor cells by laser capture microdissection is a powerful and robust method to rescue tumor cytological specimens with a low tumor cell proportion for DNA-based analysis. The fact that cells on smears, cytospins, or liquid-based cytology (LBC) preparations are intact makes them also an ideal substrate for the analysis of chromosome and gene copy numbers or gene rearrangements by fluorescence in situ hybridization. In contrast, the great variability of preparation, fixation, and staining methods in cytology is a hurdle toward standardized protocols for predictive immunohistochemistry. This can be partly overcome by cell blocks that can be processed like formalin-fixed and paraffin-embedded tissue specimens for biomarker testing. Although there is need for more standardization in protocols of molecular testing on cytological specimens, the utility of cytology for biomarker testing is now undisputed.