Abstract Abstract 3578 Introduction: Chronic lymphocytic leukemia (CLL) patients bearing 13q14 deletion are known to experience a more favorable clinical course. Recent studies, focusing on patients with loss of 13q as the sole cytogenetic aberration at diagnosis (del13q-only cases), showed that the number of malignant cells carrying this genetic lesion correlates with a more aggressive clinical behavior. However, whether the size of the 13q deletion may also influence the clinical outcome remains to be elucidated. Patients and Methods: Probes for chromosome 13q (LSI-RB1, LSI-D13S319), 11q (LSI-ATM), 17p (LSI-p53) and chromosome 12 (CEP12) were utilized on nuclei collected at diagnosis from: i) a multi-institutional CLL cohort (342 del13q-only cases) and ii) a consecutive unselected single-institution cohort of 265 cases. RB1 deleted cases (delRB1) were defined as having at least 5% of deleted nuclei. Time to treatment (TTT) intervals, as well as Rai staging, IGHV mutational status, CD38 and ZAP70 expression, B2-microglobulin levels, all evaluated at diagnosis, were also available for all cases that entered the study. Genome wide DNA profile was performed in a pilot series of 90 CLL samples using Affymetrix GeneChip Human SNP6 arrays. Results: According to genome wide DNA analysis, delRB1 occurred in a proportion of del13q-only cases (36/90; 40%), always comprising the deleted region detected with the LSI-D13S319 probe (that covers the miR-15a/16-1 cluster and the DLEU2 gene) and characterized by a larger chromosome loss (median size 2.07 Mb vs. a median size of 0.86 Mb for the canonical del13S319). Maximally selected log-rank statistics identified the 70% of nuclei bearing del13S319 as the most appropriate cut-off value capable of separating del13q-only cases into two subgroups with different TTT distributions. Consistently, del13q-only cases with at least 70% of nuclei bearing del13S319 showed a significantly shorter TTT than del13q-only cases with less than 70% deleted nuclei (p=0.0001). Del13q-only cases were then divided in four subsets according to the percentage of nuclei bearing del13S319 with or without a concomitant delRB1: del13S319 <70% (group 1), 144 cases; del13S319 <70% + delRB1 (group 2), 95 cases; del13S319 >70% (group 3), 64 cases; del13S319 >70% + delRB1 (group 4), 39 cases. The median TTT of group 1 (not reached) was significantly longer than the median TTT of group 2 (92 months, p=0.012), group 3 (68 months, p<0.0001), and group 4 (82 months, p=0.0025; see Fig. 1A). Multivariate Cox proportional hazard analyses selected the presence of delRB1 (p=0.029), along with the IGHV mutational status (p<0.0001), as an independent negative prognosticator in the context of del13q-only cases with low/intermediate Rai risk (Rai stage of 0/I at diagnosis) and <70% of del13S319. Cases belonging to the consecutive unselected single-institution CLL cohort were divided into subsets according to the classification proposed by Döhner et al (NEJM, 2000). Notably, the presence of del13S319 in <70% of cells in the absence of delRB1 identified a patient subset with particularly stable and benign clinical course (group A in Fig. 1B, 48 cases; median TTT not reached). Conversely, patients characterized by del13S319 in <70% of cells but with a larger deletion, as determined by concomitant delRB1 (group B, 24 cases), or del13S319 in >70% of cells (with or without delRB1, group C, 25 cases) or a normal karyotype (group D, 75 cases) had shorter median TTT intervals (ranging from 105 to 129 months, p<0.01 in all the comparisons). Finally, patients affected by CLL bearing trisomy 12 (group E, 48 cases) and del11q or del17p (group F, 45 cases) experienced the worst clinical courses (p<0.0001). Conclusion: In the context of del13q-only cases, different clinical outcomes were associated to the percentage of 13q14 deleted cells, as well as to the size of the 13q14 deletion, as detected by the LSI-RB1 probe. Moreover, the presence of delRB1 emerged as a feature capable of refining the prognostic assessment in the context of CLL cases with <70% del13S319. The underlying genetic mechanisms correlated with the different clinical outcomes and associated with the size of the 13q deletion are presently under investigation. Disclosures: No relevant conflicts of interest to declare.