Circulating tumor cells and "suspicious objects" evaluated through CellSearch® in metastatic renal cell carcinoma

Background: Recent evidence supports the hypothesis that the CellSearch assay, used in the enumeration of circulating tumor cells (CTCs), may underestimate the number of CTCs, especially in tumors, such as renal cell carcinoma, frequently lacking cytokeratin expression. According to the CellSearch guidelines, all objects with no clear cytokeratin staining are defined as "suspicious objects", and are not counted as CTCs. The aim of this study was to investigate the presence of CTCs and "suspicious objects" in 25 patients affected by metastatic renal cell carcinoma (mRCC). Patients and Methods: Twenty-five patients were enrolled in the study, all with a diagnosis of metastatic clear cell RCC. The CellSearch™ system was used to count the CTC in 7.5 mL of whole blood. A further 10 ml blood obtained from each patient was used to isolate CTCs through CELLection™ Dynabeads®. The expression of cytokeratin (CK) 8, 18, 19 and CD44 were evaluated by RT-PCR. Results: Standard CTCs and suspicious objects were found in 16% and 60% of the patients, respectively. CK-8/18/19 transcripts were found in 15% and CD44 in 68% of the 19 patients with evidence of classical CTC or "suspicious objects" as assessed by Cellsearch. Conclusion: The low number of CTCs detected through CellSearch in renal cell carcinoma may be due to the presence of a CTC population with atypical characteristics and a peculiar gene expression profile, characterized by lack of cytokeratin expression and gain of CD44.