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Introduction: Transformation of Marginal Zone Lymphoma (MZL) into an aggressive histology is uncommon phenomenon that can occur at any time after diagnosis and is expected to have a detrimental impact on prognosis. Biological and clinical knowledge on transformed Marginal Zone Lymphoma (tMZL) is poor and no standard treatment is established in the Rituximab era for these patients (pts). We retrospectively analyzed all consecutive biopsy proven tMZL in two Italian Hematological Divisions from 2002 to 2014 and we focused on post-transformation treatment and outcome. Methods: The dataset included 378 MZL pts diagnosed between 2002 and 2014 at Division of Hematology in Pavia and in Milan (Niguarda Hospital): 204 pts (53.9%) by extranodal MALT lymphomas, 113 pts (29.8%) by splenic MZL, and 61 pts (16.3%) by nodal MZL. Histological transformation (HT) was defined as transformation into an aggressive lymphoma at any time from previous MZL diagnosis; only cases with biopsy confirmed HT were comprised in the present analysis, while cases with only clinical suspicion of transformation were not counted as tMZL. Results: HT was documented in 18 of the 378 pts (4.8%), 6.5% in nodal MZL, 7.0% in splenic MZL and 2.9% in MALT. Histology at transformation was Diffuse Large B-Cell lymphoma in all but one case; the remaining pt was diagnosed as high-grade B-cell lymphoma, unclassifiable. CD20 was negative only in one Rituximab-naïve pt. Median time from first diagnosis to HT was 31 months (range: 10-124) and median number of previous therapies was 1 (range 0-1); pts received first line therapy listed in table 1. Median age at transformation was 68 years (range: 46-85), M/F ratio was 0.8. In the tMZL population, first diagnosis was nodal MZL in 4 pts (22%), splenic MZL in 8 pts (45%) and MALT in 6 pts (33%). At first diagnosis of MZL, 72% of t-MZL pts had stage IV disease, 17% had B symptoms, 11% had elevated LDH and ECOG performance status was lower than 2 in all the cases. HCV serology was positive in 5/17 cases; HCV status was not available for one pt. At HT disease stage was III or IV in 14 pts (78%), B symptoms were present in 7 pts (39%), LDH and beta2microglobulin were both elevated in 7 pts (39%) and ECOG performance status was lower than 2 in all the cases. Pts received post-HT treatment listed in table 2. At time of analysis 6 pts died (33%), and the main cause of death was progressive disease. With a median post-transformation follow-up of 16.6 months (range: 2-98), the 2-years Progression-Free Survival (PFS) was 45,4 % and the 2-years Overall Survival (OS) was 56.75%. No correlation was found between the following characteristics and survival: MZL type at first diagnosis, stage, symptoms, LDH and ECOG at HT, number and types of pre-HT therapies. Conclusions: This large cohort confirms that HT is a relatively rare and early event in MZL. At present time, we did not identify any feature predictive of outcome for transformed MZL. Chemotherapy in combination with Rituximab showed to be an effective treatment for tMZL. Table 1 First line treatment N. of patients % Therapy strategy CVP 7 39 Anthracycline-containing regimen 2 11 Chlorambucil monotherapy 5 28 Splenectomy 1 5.5 H.pylori eradication 1 5.5 Watch and wait strategy 2 11 Rituximab incorporation Included in first line therapy 6 33 Maintenance 0 0 Response ORR (%) 67 CRR (%) 33 Table 2. Post-HT treatment N. of patients % Therapy strategy CHOP regimen 16 89 Platinum-containing regimen 1 5.5 Missing 1 5.5 Rituximab incorporation Included in first line therapy 17 94 Maintenance 0 0 ASCT consolidation 3 17 Response ORR (%) 61 CRR (%) 59 Disclosures Rusconi: Roche: Honoraria.