Abstract P1-04-16: Multi-center experience with CELLSEARCH® circulating tumor cell kit on patients with metastatic breast cancer
Authors: Montero AJ , Eapen S , Tran KN , Gorin B , Adler P 
Published: August 2014

Background: Clinical studies have shown that circulating tumor cell (CTC) detection via the CELLSEARCH® assay is a validated prognostic marker for the prediction of progression-free survival (PFS) and overall survival (OS) in metastatic breast cancer (mBC) patients. We evaluated the impact of CTC testing on OS using real-world observational data from multiple centers.

Material and Methods: Electronic medical records (EMR) were used to identify patients who were: (i) women &gt; 18 years diagnosed with mBC on or after their date of registration, (ii) registered to the EMR system on or after 4/1/2010, and (iii) followed for at least 2 months from the time of metastases. Patients who had CTC testing were required to have at least 1 valid CTC test result. OS among mBC patients with CTC testing was compared to a cohort of patients without CTC testing (non-CTC) using a Cox model controlling for age, race, hormone receptor status, and scanning frequency.

Results: CTC patients were treated in 43 centers and grouped into two main sites based on region (N1 = 123, N2 = 240); non-CTC patients were treated in 58 different centers (N = 1,115). Mean (SD) ages of CTC and non-CTC patients were 66 (13) and 65 (14) years, respectively. ER or PR+ patients comprised 77% of the CTC tested and 72% of the non-CTC samples. The majority of patients in both cohorts were Caucasian (83% - CTC, 57% - non-CTC). A Cox model comparing OS between CTC and non-CTC patients did not show a survival benefit due to CTC testing. Comparing CTC testing frequency between the two sites showed that, on average, the 123 patients from site 1 were tested more frequently than the 240 patients in site 2 (0.6 vs. 0.3 CTC tests/month, p-value&lt;0.001). The model that compared OS between CTC patients from site 1 showed a 43% reduction in risk of death (HR = 0.58, p-value = 0.005) compared to the overall cohort of non-CTC patients. Patients from site 2 did not show a reduction in risk of death compared to non-CTC patients, but a significant interaction between CTC testing and race (Caucasian vs. non-Caucasian) (p-value = 0.005) was observed. For Caucasians, CTC patients exhibited a 37% reduction in risk of death compared to non-CTC patients; reduction in risk of death was not observed for non-Caucasians CTC patients. This interaction (p-value = 0.005) was consistent among the subset of ER or PR+ Caucasian patients relative to non-CTC ER or PR+ Caucasian patients (HR = 0.55). For site 1, the subset of ER or PR+ CTC also had a lower risk of dying compared to ER or PR+ non-CTC (HR = 0.47, p-value = 0.004).

Discussion: These results suggest that frequent serial testing for CTC may be associated with better prognosis compared to no CTC testing or even infrequent CTC testing. By more frequent monitoring, the assessment of serial CTCs provide clinicians with more frequent information, which in turn can provide a clearer understanding of a patient's cancer progression in a metastatic setting. One limitation of these results is the difficulty in ruling out other potential drivers. Particularly, results regarding the racial differences in outcomes where race might serve as a proxy for a number of factors such as severity of disease which is difficult to derive from a retrospective observational database.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P1-04-16.