Abstract
BackgroundCirculating tumour cells (CTC) are receiving increasing attention as prognostic, predictive and pharmacodynamic biomarkers in cancer patients. However, their clinical significance can be dependent on an accurate determination of CTC around cut-off values at low cell counts (<10 cells/7.5 ml). Consequently, we have conducted method validation of the CellSearch™ system focusing on clinical samples containing CTC in the cut-off region.MethodsAnalytical accuracy was first assessed employing quality controls (QC) and spiked healthy volunteer blood specimens. Results were analysed by β-expectation tolerance intervals (BETI). Inter-operator error (6 different readers) was then characterised in 38 different patient samples, 68% of which had ≤5 CTC and data were analysed by β-content γ-confidence tolerance intervals (BCTI).ResultsResults from QCs and spiked blood confirmed a 3-4-fold higher degree of imprecision at the low (48 cells, BETI = + 0.288/-0.345, β = 95%) compared to the high QC (987 cells, BETI = +0.065/-0.140, β = 95%). However, when data for individual analysts were interrogated characteristic systematic errors were detected. In the analysis of patient samples again individual analysts introduced a highly specific error into the interpretation of CTC images, which correlated to the level of training and experience. When readers were selected based on BETI and BCTI results, the high level of between-operator error (up to 170%) observed at CTC of ≤ 5 was reduced to < 30%.ConclusionsInter-operator variability in enumeration of CTC at low cell counts can be considerable, but is also potentially avoidable by following simple guidance steps.
Highlights
Circulating tumour cells (CTC) are receiving increasing attention as prognostic, predictive and pharmacodynamic biomarkers in cancer patients
Patients and blood sample collection Blood samples (7.5 ml) for CTC enumeration were collected from a total of 38 different lung, prostate, melanoma and colorectal patients receiving standard of care chemotherapy at the Christie Hospital, Manchester and entered into a number of experimental medicine studies being conducted at the Paterson Institute for Cancer Research
The aim of the present study was to improve the accuracy of CTC enumeration by the CellSearch system at low cell counts (
Summary
Circulating tumour cells (CTC) are receiving increasing attention as prognostic, predictive and pharmacodynamic biomarkers in cancer patients. CTC are believed to act as the ‘seeds’ for the establishment of metastatic disease, and a mechanism to re-populate the primary tumour, and their presence has been shown to correlate to both progression free survival and overall survival [2,3,4,5] In these studies, a discrete cut-off point was discriminated at extremely low numbers of CTC above which poorer prognosis was observed: ≥ 5 in 7.5 ml blood for metastatic breast cancer, metastatic castration resistant prostate cancer and non-small cell lung cancer; ≥ 3 for metastatic colorectal cancer; ≥ 2 in melanoma and ≥ 1 in neuroendocrine tumours [2,3,4,5,6,7,8]. CellSearch remains the benchmark against which all new technologies should be assessed [18]
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