The significance of circulating tumor cells in prostate cancer patients undergoing adjuvant or salvage radiation therapy

Background: Following radical prostatectomy, success of adjuvant and salvage radiation therapy (RT) is dependent on the absence of micrometastatic disease. However, reliable prognostic/predictive factors for determining this are lacking. Therefore, novel biomarkers are needed to assist with clinical decision-making in this setting. Enumeration of circulating tumor cells (CTCs) using the regulatory-approved CellSearch System (CSS) is prognostic in metastatic prostate cancer. We hypothesize that CTCs may also be prognostic in the post-prostatectomy setting. Methods: Patient blood samples ( n =55) were processed on the CSS to enumerate CTCs at 0, 6, 12 and 24 months after completion of RT. CTC values were correlated with predictive/prognostic factors and progression-free survival. Results: CTC status (presence/absence) correlated significantly with positive margins (increased likelihood of CTC neg disease; P =0.032), and trended toward significance with the presence of seminal vesicle invasion (CTC pos ; P =0.113) and extracapsular extension (CTC neg ; P =0.116). Although there was a trend toward a decreased time to biochemical failure (BCF) in baseline CTC-positive patients ( n =9), this trend was not significant (hazard ratio (HR)=0.3505; P =0.166). However, CTC-positive status at any point ( n =16) predicted for time to BCF (HR=0.2868; P =0.0437). Conclusions: One caveat of this study is the small sample size utilized ( n =55) and the low number of patients with CTC-positive disease ( n =16). However, our results suggest that CTCs may be indicative of disseminated disease and assessment of CTCs during RT may be helpful in clinical decision-making to determine, which patients may benefit from RT versus those who may benefit more from systemic treatments.