P046 | Beyond the mutation: prognostic role of FLT3-ITD length and genomic context in acute myeloid leukemia

Acute Myeloid Leukemia (AML) is a genetically heterogeneous malignancy characterized by recurrent somatic mutations that influence disease biology and prognosis. Among these, internal tandem duplication (ITD) mutations in the FLT3 gene are among the most frequent and clinically adverse alterations. However, the prognostic relevance of ITD length and insertion site remains controversial, highlighting the need for further investigation. We conducted a single-center observational study including 384 newly diagnosed adult AML patients (pts) over 5 years, analyzed both prospectively and retrospectively. The FLT3-ITD mutation was detected in 72 pts (19%) by NGS or RT-PCR. ITD length (base pairs, bp) was correlated with clinical and biological features, and an optimal threshold was determined by Maximally Selected Rank Statistics (MAXSTAT). For 37 pts (51%) evaluated by NGS, the precise genomic insertion site was defined its biological and clinical relevance. The median age at diagnosis was 64 years. Co-mutations most frequently involved NPM1 (59%) and DNMT3A (43%). Multiple ITDs were detected in 6 pts. The median ITD length was 48 bp (IQR 30-69). Most insertions affected exon 14 (95%), while exon 15 and intronic regions were less commonly affected (5 and 16%, respectively). MAXSTAT analysis defined 63 bp as the optimal cut-point (Figure 1A), defining two distinct prognostic groups with comparable baseline features.Pts who relapsed showed significantly longer ITDs (median 71 vs 42 bp; p=0.018) (Figure 1B). A higher 5-year Cumulative Incidence of Relapse was observed in pts with long ITD (≥63 bp) (sHR=3.57; 95% CI:1.21–10.56; p=0.0216) (Figure 1C), with a consistent trend toward worse Overall and Event Free Survival. The negative prognostic effect of longer ITDs persisted after adjustment for age (p=0.0003), supporting ITD length as an independent predictor of outcome.Longer ITDs were more frequently associated with intronic involvement (p=0.001). No correlation was found between length and total number of co-mutations; however, DNMT3A-mutated pts displayed significantly longer ITDs and a fivefold higher likelihood of harboring ITDs ≥63 bp. Notably, pts harboring both DNMT3A mutation and long ITD (≥63 bp) (27%) had markedly reduced 5-year OS (p=0.02) (Figure 1D). Within the DNMT3A-mutated subgroup, longer ITDs also conferred inferior survival (p=0.03) (Figure 1E).Our findings demonstrate that longer FLT3-ITDs are associated with higher relapse risk and poorer clinical outcomes. We also identify a prognostically adverse interaction between DNMT3A mutation and ITD length. The preferential intronic involvement of long ITDs may indicate functional diversity with biological impact. These findings support integrating of ITD length, insertion site, and co-mutational status, particularly DNMT3A, into prognostic algorithms to improve risk stratification and guide treatment choice, including therapy intensification and indication for allogenic stem cells transplantation.