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ALL is the most common childhood malignancy and the leading cause of pediatric cancer-related mortality. Minimal residual disease (MRD) is a powerful predictor of outcome in childhood ALL and allows stratification of patients into prognostic risk subgroups and the application of risk-directed therapy. MRD assessment is routinely performed by flow cytometry and quantitative real-time polymerase chain reaction (qRT-PCR).

a descriptive study of childhood ALL diagnosed in a single center between September 2013 and February 2015 and treated according to Spanish Pediatric Hemato-Oncology Society SEHOP-PETHEMA-2013 protocol. The initial diagnosis was established with the monoclonal antibodies panels proposed by the EuroFlow Group. Detection of MRD was based on leukemia-associated immunophenotype (LAIP) observed at diagnosis. Fusion genes ETV6-RUNX1, TCF3-PBX1 and BCR-ABL1 were analyzed by qRT-PCR following the guidelines of the Europe Against Cancer program.

twenty-six patients were included (17 males, median age 6.5 years, range 1.1 - 16.7 years). Twenty-one cases were B-cell precursor ALL (B-pre-ALL) in common stage (CD10+) and 5 patients had T-cell ALL in cortical (CD1a+) stage. The most frequent LAIPs identified in B-pre-ALL were cross-lineage antigen expression (studying CD13, CD15, CD123 and CD66) and asynchronous antigen expression. ETV6-RUNX1 rearrangement was detected in six patients and TCF3-PBX1 rearrangement was observed in three cases. All patients had a good response to prednisone at day +8 and the evaluation on day +15 by morphology showed persistent disease (≥5% blasts) in 4 B-pre-ALL cases. At the end of induction IA all patients (24 cases evaluated) achieved morphologic remission (<5% blasts). The MRD by 8-MCF on day +15 was negative (<0.01%) in 8/26 cases (5 B-pre-ALL and 3 T-ALL), on day +33 in 21/24 cases and on day +78 in all cases (18 cases evaluated). Two follow-up samples were not evaluable by qRT-PCR analysis. The MRD study by qRT-PCR was concordant with the 8-MFC in 17/19 samples. In a patient with TCF3-PBX1 rearrangement the MRD on day +33 was negative by 8-MCF but positive by qRT-PCR (ratio TCF3-PBX1/ABL x10⁴ 3 copies) but the assessment on day +78 was negative in both. In addition, a patient with ETV6-RUNX1 rearrangement had a negative MRD on day +33 by 8-MCF but qRT-PCR still detected a very low number of copies (ratio ETV6-RUNX1/ABL x10⁴ 4.9 copies). The evaluation on day +78 for this patient is still pending.

implementation of MFC-8 has optimized the diagnosis and monitoring of MRD using less amount of sample for the analysis, representing a significant improvement in pediatric care. Furthermore, a good correlation of the MFC-8 MRD levels with molecular results was observed (89.5% of cases), suggesting a good complementarity of both techniques. Thus, MFC-8 confirms as a simple, fast and very useful tool to diagnose and follow MRD with high sensitivity in pediatric ALL patients.