Outcomes of patients treated for monomorphic B-cell post-transplant lymphoproliferative disorder: A single-center experience.

7069 Background: Patients (pts) with monomorphic B-cell post-transplant lymphoproliferative disorder (B-PTLD) have poor outcomes and high treatment-related mortality (TRM). With risk-stratified sequential treatment (RSST), some can be treated with rituximab alone. We sought to evaluate predictors of outcome in newly diagnosed B-PTLD, including involvement of extranodal (EN) sites and allograft. Methods: Adults diagnosed at Siteman Cancer Center with confirmed B-PTLD from 1/1/2006–12/31/2024 were identified via electronic health records. Kaplan-Meier and log-rank tests were used to evaluate progression-free survival (PFS) and overall survival (OS). Univariate analyses were done using Fisher’s exact and χ2 tests with a=0.05. Results: 106 pts with monomorphic B-PTLD were identified (Table). The most common graft types were kidney (33%), liver (26%), and lung (15%). After diagnosis, immunosuppression was reduced in 93 (88%) pts and 105 pts received systemic therapy. 56 (53%) pts received frontline chemotherapy (chemo), most commonly R-CHOP (68%, n=38) and DA-EPOCH-R (21%, n=12), and overall response rate (ORR) was 66% (complete response [CR] 57%). 49 (46%) pts initially received rituximab monotherapy, including 39 on RSST, with ORR 36% (CR 23%). 28 (72%) were escalated to chemo, including 7 (18%) pts who progressed prior to reimaging. Treatments used were R-CHOP (71%, n=20), Pola-R-CHP (21%, n=6), and DA-EPOCH-R (7%, n=2), with ORR 59% (CR 56%). With median follow up of 58 months (mo), median PFS and OS were 33 and 64 mo, respectively. Pts who did not have CR to rituximab had worse PFS vs responders (12 mo vs 86 mo, p=0.037) and OS (27 mo vs 86 mo, p=0.04). Factors predicting CR to rituximab included early stage (p=0.001) and absence of EN disease (p=0.005). Graft type, involvement of allograft, tumor EBV status, and gene rearrangements were not predictive. Following frontline therapy, 33 (31%) pts developed relapsed/refractory disease. Elevated LDH (p=0.021) and extranodal disease (p=0.032) at diagnosis predicted relapse. 13 (12%) pts experienced graft failure, and 6 (6%) required repeat transplant. Discontinuation of antimetabolites was associated with higher rates of graft failure (p=0.020). 51 (48%) pts died, 8 (16%) due to TRM. Conclusions: In a large cohort of B-PTLD, ORR to rituximab on RSST and chemo confirm prior data. Pts with lack of response to rituximab monotherapy, EN disease, and elevated LDH have worse outcomes. Despite advances, TRM remains high in PTLD. Demographics and predictors of response to rituximab. Variable Median (range) Variable n (%) OR (response to rituximab) p-value Age 60 (22-86) Male 66 (63) 1.7 0.65 Years since transplant 8 (0-34) White 92 (87) 1.7 0.66 Non-GCB 47/85 (55) 1.4 0.68 EBV-neg 77/101 (76) 3.7 0.087 Allograft involved 19 (18) 1.0 0.98 FISH: MYC 20 (19) 2.4 0.46 MYC + BCL2 1 (1) NA NA Early stage 15 (14) 0.04 0.001 EN disease 85 (80) 0.14 0.005