AbstractBackgroundCirculating endothelial cells (CECs) have been detected at increased numbers in patients with solid cancers. CECs have not been systematically evaluated in patients with osteosarcoma.ProcedurePatients 12 months to 30 years of age with newly diagnosed high‐grade osteosarcoma were eligible for this prospective cohort study. Patients provided a single blood sample at study entry for CEC quantification by flow cytometry at a single reference laboratory. CECs were defined as CD146+, CD31+, CD45−, and CD133−. CEC progenitor cells (CEPs) were defined as CD146+, CD31+, CD45−, and CD133+.ResultsEighteen patients enrolled (11 males; median age 16 years; range 5–21 years). CEC counts did not differ between patients with osteosarcoma compared to seven pediatric healthy controls (median 645 cells/ml, range 60–5,320 cells/ml vs. 1,670 cells/ml, range 330–4,700 cells/ml, respectively; P = 0.12). CEP counts did not differ between patients compared to controls (median 126 cells/ml, range 0–5,320 cells/ml vs. median 260 cells/ml, range 0–10,670 cells/ml, respectively; P = 0.69). CEC and CEP counts did not correlate with metastatic status, tumor size, or histologic response to neoadjuvant chemotherapy.ConclusionsCEC and CEP levels are not increased in patients with osteosarcoma compared to healthy controls. CECs and CEPs do not correlate with clinical features of osteosarcoma. Alternative novel markers of disease burden and response are needed in this disease. Pediatr Blood Cancer 2012; 58: 181–184. © 2011 Wiley Periodicals, Inc.