Impact of Two Measures of Micrometastatic Disease on Clinical Outcomes in Patients with Newly Diagnosed Ewing Sarcoma: A Report from the Children's Oncology Group

Purpose: Flow cytometry and RT-PCR can detect occult Ewing sarcoma (ES) cells in the blood and bone marrow (BM). These techniques were used to evaluate the prognostic significance of micrometastatic disease in ES. Experimental Design: Newly diagnosed patients with ES were enrolled on two prospective multi-center studies. In the flow cytometry cohort, patients were defined as "positive" for BM micrometastatic disease if their CD99+/CD45- values were above the upper limit in 22 control patients. In the PCR cohort, RT-PCR on blood or BM samples classified the patients as "positive" or "negative" for EWSR1/FLI1 translocations. The association between micrometastatic disease burden with clinical features and outcome was assessed. Co-expression of IGF-1R on detected tumor cells was performed in a subset of flow cytometry samples. Results: The median total BM CD99+CD45- percent was 0.0012% (range 0-1.10%) in the flow cytometry cohort, with 14/109 (12.8%) of ES patients defined as "positive." In the PCR cohort, 19.6% (44/225) patients were "positive" for any EWSR1/FLI1 translocation in blood or BM. There were no differences in baseline clinical features or event-free or overall survival between patients classified as "positive" vs. "negative" by either method. CD99+CD45- cells had significantly higher IGF-1R expression compared to CD45+ hematopoietic cells (mean geometric mean fluorescence intensity 982.7 vs. 190.9; p<0.001). Conclusions: The detection of micrometastatic disease at initial diagnosis by flow cytometry or RT-PCR is not associated with outcome in newly diagnosed patients with ES. Flow cytometry provides a tool to characterize occult micrometastatic tumor cells for proteins of interest.

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