P025 | SUBDIAPHRAGMATIC LOCALIZATION AND POST-INDUCTION DS5-PET SHOWING PROGRESSIVE DISEASE ARE THE MOST SIGNIFICANT PREDICTORS OF POOR PROGNOSIS IN PATIENTS WITH PRIMARY MEDIASTINAL B-CELL LYMPHOMA: A SINGLE-CENTRE EXPERIENCE

Primary Mediastinal B-cell Lymphoma (PMBCL) is a rare aggressive B-cell lymphoma. Although the prognosis has improved with the use of anthracycline-based chemotherapy combined with rituximab, about 10% of patients (pts) experience refractory disease or early treatment failure, which correlate with a poor prognosis. This study aimed to identify unfavourable characteristics that can predict adverse outcome in PMBCL. We retrospectively analysed PMBCL pts under 66 years (y) diagnosed between 2008 and 2023. A total of 99 pts were included (characteristics in Table 1). Treatment was based on the IPI score: pts with L/IL IPI score received R-MegaCHOP14x6 followed by consolidation radiotherapy to the bulky mediastinal site; pts with H/IH IPI score underwent ASCT consolidation after induction with R-CHOP14x4 + R-MADx2 or R-CHOP14x6 + R-AraC, according to age under or over 60 respectively. We analysed overall survival (OS) and progression-free survival (PFS) based on IPI score, FIL score and disease localization. Regarding IPI score, no statistically significant differences in survival were observed between L/LI and H/IH groups: 2-y PFS and OS were respectively 94,0% vs 85,7% (p=0.395) and 100% vs 91,8% (p=0.526). The FIL score identified a group of pts with worse 2-y PFS (L-risk: 95,0% vs I-risk: 93,2% vs H-risk 75,0%; p=0.036) without significant difference in 2y-OS (L-risk: 100% vs I-risk: 98,3% vs H-risk 85,0%; p=0.203). Pts with above diaphragm (AD) localization had significantly better 2-y PFS (93,8% vs 82,4%; p=0.047) and 2-y OS (100% vs 88,2%; p=0.024) compared to those with above and below diaphragm (BD) localization. A total of 86 pts had a PET post-induction therapy evaluated according to Deauville Score (DS). No statistically significant prognostic difference was observed between 42 pts with negative PET (DS1-3) and 28 pts with DS4 (2-y OS 97,6% vs 100% respectively; p=0.788). The 16 pts with DS5 had a significantly worse 2-y OS compared to pts with DS1-4 (81,3% vs 98,6% respectively; p=0.009). However, in the DS5 group, pts in partial remission (n=11) exhibited a significantly superior 2-y OS compared to those with progressive disease (n=5) (90,9% vs 60,0% respectively; p=0.041). In conclusion, among PMBCL pts, BD localization and post-induction PET, specifically DS5 with evidence of progressive disease, represent significant prognostic indicators of poor survival. Future studies should explore novel treatment strategies for these high-risk PMBCL pts.