P362: MINIMAL-LOSS-CYTOMETRY FOR EVALUATION OF CEREBROSPINAL FLUID IN PEDIATRIC ACUTE LYMPHOBLASTIC LEUKEMIA. SINGLE CENTER STUDY. (2022)

Background: Central nervous system (CNS) involvement at diagnosis of acute lymphoblastic leukemia (ALL) represents a risk factor for CNS relapse. Currently, examination of the cerebrospinal fluid (CSF) is performed and evaluated quantitatively using a counting chamber and qualitatively using cytomorphological analysis of cytospin. Recently, flow cytometry (FC) evaluation of CSF was shown to provide more reliable results and to predict relapse risk (de Haas et al., 2021, Thastrup et al., 2020). Independently of these studies, we applied a lyse-no wash technique aiming at minimizing cell loss during preparation and at improved cell concentration estimate. Aims: Our aim was to compare our findings with morphological criteria and published data and correlate them with clinical outcome in pediatric patients with ALL. Methods: We included pediatric patients with newly diagnosed ALL (04/2014 – 01/2022), whose CSF examination was performed in CLIP laboratory (n=208). Patients were classified as B cell precursor ALL (BCP ALL; 85%) and T ALL (15%) and treated according to following protocols: AIEOP-BFM ALL 2009 (n=118), AIEOP-BFM ALL 2017 (n=71), EsPhALL(n=8), Interfant06 (n=5), off-protocol (n=6). During follow up, 14 patients relapsed (5 CNS incl. combined, 9 non-CNS). CSF (drawn into tube with no fixative agent and processed within 2 hours) was concentrated by centrifugation and incubated with combination of antibodies (CD20/CD10/CD45/CD34/CD19/CD3/Syto41 in BCP ALL and CD4/CD99/CD5/CD3/CD7/CD16 + 56/CD8/CD45/Syto41 in T ALL). Residual erythrocytes were lysed with NH4Cl solution, followed by immediate acquisition on BD FACS Lyric or BD LSRII cytometers. Blasts were quantified to initial CSF volume. Cluster of 20 events was required to assign samples as FC positive (FC+). In parallel, CNS status was concluded according to morphological criteria, being classified as CNS1 (no blasts), CNS2a/b (≤5 WBC/μL with blasts), CNS2c/CNS3 (≥5 WBC/μL with blasts) according to treatment protocol guidelines. Continuous variables were compared with the Mann–Whitney test, survival data were analyzed using Mantel-Cox test. P values<0.05 were considered as significant. Results: Using FC, we identified atypical blasts in 50 out of the 208 analyzed samples (24%). Median of analyzed CSF volume was 367µL (range 143-1033µL), which enabled median sensitivity 0.054 events/µL (range 0.02-0.14ev/µL). Mean blast concentration was 0.47ev/µL (range 0-25ev/µL) in all samples and 2 ev/µL (range 0.01-25ev/µL, median 0.53ev/µL) in FC+ samples. We observed significantly higher blast concentration in patients with T ALL, in patients with CNS2 and CNS3 and those who subsequently relapsed. Patients with FC+ had significantly lower relapse-free survival than FC- patients (55% vs 94%), which provided better separation than CNS status (68% vs.85% for CNS2/3 vs. CNS1, respectively). Image:Summary/Conclusion: Using fresh CSF sample with lyse-no wash protocol enabled us to detect approximately 10x higher blast count (0.24ev/µL in FC+CNS1 and 1.25ev/µL in FC+CNS2/3) than was recently published (0.015ev/µL in FC+CNS1 and 0.073ev/µL in FC+CNS2/3 in Thastrup et al., 2020), and thus it better correlates to cytomorphological data. Interestingly, the proportion of FC+ patients in our cohort is comparable to (24% vs 25%), showing that although in our method we lose fewer cells, we identify the same proportion of FC+ patients. The presented study shows a higher impact of FC positivity on relapse risk than described previously. The disadvantage of our method is a need for a fresh sample, which is investigated locally. Supported by UNCE/MED/015, NU20J-07-00028.