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Background: Relapse remains a serious event and main obstacle to permanent cure in childhood acute myeloid leukemia (AML). Reduction of measurable/minimal residual disease (MRD) assessed by real-time quantitative PCR (qPCR) during therapy is predictive of outcome in adults but persistent MRD positivity may be observed despite long-term remission and hamper accurate risk assignment. Sequential MRD determinations, rather than analysis at single landmark time points, may better capture the dynamics of leukemia eradication and identify relapse at molecular levels when applied in the post-therapy setting. We investigated the post-induction qPCR MRD kinetics in peripheral blood (PB) and bone marrow (BM) in a large cohort of childhood AML patients and demonstrate the utility of serial assessments for disease surveillance after therapy completion. Methods: We collected the results from qPCR MRD analyses of 774 samples (BM 347, PB 427) from 75 children with AML harboring recurrent fusion transcripts (32 RUNX1-RUNX1T1, 24 CBFB-MYH11, 16 KMT2A-MLLT3 and 3 KMT2A-ELL). Patients were treated according to The Nordic Society for Paediatric Haematology and Oncology (NOPHO)-AML 2004 or NOPHO-DBH AML 2012 protocols (2004 - 2016), or AML-BFM 2012 protocol (2014 - 2016). Only patients who achieved complete remission (CR) during induction therapy and received standard-risk consolidation without allografting were eligible for the study. Risk of relapse as a function of time of MRD positivity in sequential samples from BM or PB during consolidation therapy was modeled using restricted cubic splines. Considering shifting from MRD negative to positive in PB and MRD increments in BM above 5x10-4 as time-dependent variables, the Mantel-Byar method was applied to evaluate the prognostic impact of MRD kinetics during follow-up. Results: Risk of relapse was independent of MRD persistence in BM during therapy, but showed a strong correlation with time of MRD positivity in PB where 8/8 patients with detectable MRD after first consolidation course relapsed (Figure 1A and 1B). At therapy completion, MRD level in BM did not correlate to outcome, HR=0.64/MRD log reduction, 95% confidence interval (CI):0.32 - 1.26 (P=0.19), and there was no difference in 3-year cumulative incidence of relapse (CIR) according to MRD status (P=0.51, Figure 2A). Four patients remained MRD positive throughout consolidation therapy and all subsequently relapsed, whereas 7/28 patients who were MRD negative in PB at the end of therapy suffered relapse, 3-year CIR=27%, CI:14% - 49% (P Shifting from negative to positive in PB during follow-up predicted subsequent relapse in 10/10 patients. All 253 PB samples collected during follow-up from 36 patients in continuous CR were MRD negative. In core binding factor (CBF) AML, persistent low-level MRD positivity in BM was frequent (detected in 38% of patients tested within six months of therapy completion) but an increment to above 5x10-4 heralded subsequent relapse in 12/12 patients. Pre-relapse MRD kinetics were delineated in 16 patients and revealed a median log increment/30 days of 0.8 (range:0.4 - 1.5) in CBF patients versus 2.2 (range:1.1 - 3.7) in patients with KMT2A-MLLT3 (P=0.008). Perspectives: This study demonstrates that qPCR MRD monitoring in PB, rather than BM, represents an accurate discriminator of prognosis and is a highly informative tool for disease surveillance in childhood AML. Early relapse detection during follow-up may facilitate preemptive therapy strategies of molecular relapse, possibly improving relapse treatment outcomes. Disclosures No relevant conflicts of interest to declare.

Minimal residual disease (MRD) measured by PCR of clonal IgH/TCR rearrangements predicts relapse in T-cell acute lymphoblastic leukemia (T-ALL) and serves as risk stratification tool. Since 10% of patients have no suitable PCR-marker, we evaluated flowcytometry (FCM)-based MRD for risk stratification. We included 274 T-ALL patients treated in the NOPHO-ALL2008 protocol. MRD was measured by six-color FCM and real-time quantitative PCR. Day 29 PCR-MRD (cut-off 10-3) was used for risk stratification. At diagnosis, 93% had an FCM-marker for MRD monitoring, 84% a PCR-marker, and 99.3% (272/274) had a marker when combining the two. Adjusted for age and WBC, the hazard ratio for relapse was 3.55 (95% CI 1.4-9.0, p = 0.008) for day 29 FCM-MRD ≥ 10-3 and 5.6 (95% CI 2.0-16, p = 0.001) for PCR-MRD ≥ 10-3 compared with MRD

Background: PCR of rearranged antigen receptor genes is the method of choice for MRD quantification in ALL. Although FCM-MRD is faster and biologically more informative than PCR, the analysis requires a high level of training. The only larger published studies using FCM-MRD based stratification (Borowitz, Blood, 2008 and 2015) showed a clear association with clinical outcome in BCP-ALL. However, MRD analyses were centralized and these studies included only one MRD-based stratification (MRD levels at the end of induction). Patients and methods: We examined FCM-MRD as stratification tool in BCP-ALL at various timepoints in a large-scale multicenter (18 MRD centers) study. A total of 1487 patients with BCP-ALL (1298 children (younger than 18 years) and 189 adults (18-45 years) are included in the study and were treated according to the NOPHO ALL2008 protocol between July 2008 and February 2016. The median follow-up time for patients in first remission was 51 months (IQR 32-75). MRD was measured by FCM and/or real time quantitative PCR on days 15, 29 (end of induction) and 79 (for standard (SR) and intermediate risk (IR) patients) and prior to and after high risk blocks. A 6-colour FCM analysis including 3 standardized antibody combinations was used and performed in 18 laboratories. Patients were stratified by FCM-MRD, or by PCR-MRD if no FCM-MRD marker was available. End-of-induction MRD (cut-off 10-3) was used to stratify patients to standard risk (SR) vs intermediate risk (IR) or IR vs high risk consolidation therapy (in case of WBC > 100 x 109/L at diagnosis). Patients with MRD >=2.5x10-1 on day 15 were stratified to high risk block therapy. Patients with MRD >=5x10-2 on day 29 or day 79/post high risk-2 block MRD >=10-3 were stratified to HSCT. Primary outcomes were 5year event-free survival (5y EFS) and 5year cumulative incidence of relapse (5y CIR). Results: Only two patients (0.14% of total) had neither an informative FCM nor a PCR marker, and an informative FCM marker combination for MRD monitoring was identified in 96.2% of patients. There was a significant correlation between FCM- and PCR-MRD levels on day 15 (r=0.77, p Based on FCM-MRD only, the median MRD level on day 15, 29 and 79/post high risk-2 block was 5x10-3, 1.1x10-4, and below detection limit, respectively. Adults had significantly higher MRD levels at all time-points (p The day 29 FCM-MRD level was closely associated with clinical outcome and a higher hazard of relapse was seen independently for a FCM-MRD >=10-3 (hazard ratio (HR) 2.4, CI 1.6-3.7, p18 year (HR 3.0, CI 1.7-5.3, p=100 (HR 2.7, CI 1.6-4.6, p=0.0001), and B-other (HR 2.1, CI 1.2-3.5, p=0.0052) or high risk B-ALL cytogenetic aberration (rearranged KMT2A/iAMPchr21/hypodiploid) (HR 3.2, CI 1.6-6.1, p=0.0006) (multivariate cause-specific Cox regression, n=1328). Patients with a day 79 FCM-MRD >=10-4 and =10-4 and =10-4 and Patients with day 15 FCM-MRD =10-3 and Conclusion: FCM-MRD performed in a multi-center setting is a clinically useful method for disease monitoring and MRD-based treatment stratification in BCP-ALL. Moreover, FCM-MRD is a reliable indicator of outcome in BCP-ALL independently of other key risk factors. Residual disease >=10-4 and Disclosures No relevant conflicts of interest to declare.

Peripheral T-cell lymphoma (PTCL) with a nodular architecture is rare. Recently, two variants have been described with infiltration of the B-cell follicle, one variant that localizes to the marginal zone with a so-called perifollicular growth pattern, and a variant that localizes to the germinal center. These lymphomas have a CD4+ phenotype and may express Bcl-6. We have studied five similar cases of PTCL with involvement of the B-cell follicle. However, our cases differ from the cases previously described by their predominant and frequently patchy involvement of the expanded mantle zone of the B-cell follicle at onset. Later biopsies in three of the cases show diffuse infiltration of the lymph node, without features of angioimmunoblastic TCL (AILT). All cases expressed Bcl-6 in addition to CD4. Cytogenetics was available in four of the cases but revealed no recurrent chromosomal aberrations or changes associated with other types of PTCL. No mutations of the BCL-6 gene were observed. Together, the cases seem to have an intermediately aggressive clinical behavior. Whether our cases are part of a spectrum of PTCLs that encompasses previously described variants with predominant marginal zone or germinal center infiltration or they represent a separate T-cell lymphoma type remains to be demonstrated by a study of more of such cases.