Role Of Day-15 Peripheral Blood MRD Assessment In Pediatric B-ALL Patients

Minimal residual disease (MRD) has emerged as an independent prognostic factor for patients of acute lymphoblastic leukemia (ALL). There is a strong correlation between MRD levels in bone marrow and the risk of relapse in childhood & adult leukemias ^{1, 2}. Bone marrow MRD (BM-MRD) level of ≥ 0.01% is considered as positive and a mid-induction MRD of ≥ 1% is associated with high risk of relapse ³. Recently, the concept of peripheral blood MRD (PB-MRD), as a replacement for BM-MRD, has hit the lime light. In pediatric B-ALL, presence of PB-MRD is associated with a high relapse rate in comparison to cases which are PB-MRD negative ^{4, 5}.

This study was aimed to compare the levels of mid-induction (day 15) MRD levels in bone marrow and peripheral blood of pediatric B-ALL patients with a hypothesis that PB-MRD levels correlate with BM-MRD levels, and thus can predict BM-MRD levels for further management of the patient.

Forty newly diagnosed CD19⁺CD10⁺CD34^+/- pediatric B-ALL patients under Vincristine, L-Asparaginase and Dexamethasone, were assessed for MRD levels on their paired day 15 PB & BM samples using six colour flow cytometry. With informed consent, both the samples were collected in EDTA vacutainers and lyse-stain-wash technique was used to prepare a single six colour tube comprising of SYTO 13/ CD34PE/ CD20PerCP/ CD19 PECy7/ CD10APC/ CD45APCH7 for each sample. The processed samples were run on BD FACS Canto II with acquisition of 1 million events or till the tubes were empty. Analysis was done using BD FACS Diva software and MRD of ≥ 0.01% was considered positive.

Among 40 pairs of day 15 PB and BM samples, 25 (62.5%) were BM-MRD positive. Sixteen pairs (40%) had PB-MRD and BM-MRD co-positivity, 9 pairs (22.5%) had isolated BM-MRD positivity and 15 pairs (37.5%) were MRD negative in both PB and BM samples. In other words, among the 25 BM-MRD positive cases, simultaneous PB-MRD was positive in 16 patients (64%) and none of the samples had isolated PB-MRD positivity. Overall analysis of MRD positive cases showed a direct correlation between PB-MRD and BM-MRD (ρ = +0.684, p < 0.000) and BM-MRD levels were 7 times higher than the PB-MRD. In addition, ROC analysis with PB-MRD of ≥ 0.01% as a cut-off, revealed that, the most likelihood of PB-MRD being positive was when BM-MRD was ≥ 0.31%.

In contrast to the sparsely available literature, our study shows a significant correlation between PB & BM-MRD levels in day 15 paired samples of B-ALL cases. The MRD levels were 7 times higher in BM as compared to PB and PB-MRD was mostly positive with BM-MRD of ≥0.31%. In other words, day 15 PB-MRD positivity indirectly indicates that there is a minimum BM-MRD of 0.31%. Since literature reports prognostic significance of mid-induction BM-MRD at levels ≥1%, on day 15, an assessment of peripheral blood MRD alone, might yield clinically relevant prognostic information. A paired analysis at different time points might also establish a similar correlation as seen in the present study, eliminating the need of BM-MRD during further follow ups of the patient. This will help in avoiding an invasive procedure and improve patient compliance.

References

1. Irving J, Jesson J, Virgo P, Case M, Minto L, Eyre L, et al. Establishment and validation of a standard protocol for the detection of minimal residual disease in B lineage childhood acute lymphoblastic leukemia by flow cytometry in a multi-center setting. haematologica. 2009;94(6):870-4.

2. Coustan-Smith E, Sancho J, Behm FG, Hancock ML, Razzouk BI, Ribeiro RC, et al. Prognostic importance of measuring early clearance of leukemic cells by flow cytometry in childhood acute lymphoblastic leukemia. Blood. 2002;100(1):52-8.

3. Basso G, Veltroni M, Valsecchi MG, Dworzak MN, Ratei R, Silvestri D, et al. Risk of relapse of childhood acute lymphoblastic leukemia is predicted by flow cytometric measurement of residual disease on day 15 bone marrow. Journal of Clinical Oncology. 2009;27(31):5168-74.

4. Elain CS, Sancho J, Michael LH, Bassem. Use of peripheral blood instead of bone marrow to monitor residual disease in children with acute lymphoblastic leukemia. Blood. 2002;100 (7):2399-402.

5. Brisco MJ, Sykes PJ, Hughes E, Dolman G, Neoh SH, Peng LM, et al. Monitoring minimal residual disease in peripheral blood in B lineage acute lymphoblastic leukaemia. British journal of haematology. 1997;99(2):314-9.

No relevant conflicts of interest to declare.