Background:Cancer Associated Macrophage-Like cells (CAMLs) are polynucleated circulating stromal cells found in the bloodstream of numerous solid-tumor malignancies. Variations within CAML size have been associated with poorer progression free survival (PFS) and overall survival (OS) in a variety of cancers; however, no study has evaluated their clinical significance in esophageal cancer (EC). Methods:To examine this significance, we ran a two-year prospective pilot study consisting of newly diagnosed stage I-III EC patients (n=32) receiving chemoradiotherapy (CRT). CAML sizes were sequentially monitored prior to CRT (BL), ~2 weeks into treatment (T1), and at the first available sample after the completion of CRT (T2). Results:We found CAMLs in 88% (n=28/32) of all patient samples throughout the trial, with a sensitivity of 76% (n=22/29) in pre-treatment screening samples. Improved 2 year PFS and OS was found in patients with CAMLs <50μm by the completion of CRT over patients with CAMLs ≥50μm; PFS (HR=12.0, 95%CI=2.7-54.1, p=0.004) and OS (HR=9.0, 95%CI=1.9-43.5, p=0.019).Conclusions:Tracking CAML sizes throughout CRT as a minimally invasive biomarker may serve as a prognostic tool in mapping EC progression, and further studies are warranted to determine if presence of these cells prior to treatment suggest diagnostic value for at-risk populations.