Interim PET after 4 cycles predicts outcome in histomolecularly confirmed primary mediastinal B-cell lymphoma

The GAINED study was a randomized phase 3 trial comparing obinutuzumab (G) with rituximab (R) plus ACVBP (doxorubicin, cyclophosphamide, and prednisone, combined with either vindesine or bleomycin) or CHOP14 (cyclophosphamide, doxorubicin, vincristine, and prednisone, administered on a 14-day schedule) induction, followed by positron emission tomography (PET)-guided consolidation. This post hoc analysis aimed to detail the outcomes of patients with primary mediastinal B-cell lymphoma (PMBL), verified through expert pathological review and the use of gene expression profiling (GEP) and next-generation sequencing. Of 620 centrally reviewed patients, 138 (22.3%) confirmed PMBL cases were analyzed. Baseline characteristics included a median age of 33.5 years, 63.8% female, 55.1% stage III to IV, 90.6% elevated lactate dehydrogenase, 87.6% Eastern Cooperative Oncology Group performance status score of 0 to 1, 62.3% extranodal involvement, 52.6% age-adjusted International Prognostic Index (aaIPI) of 2% to 3%, and 53.6% bulk (>10 cm). Induction regimens were R/G-CHOP14 (56.9%) and R/G-ACVBP (43.1%). Postinduction treatments, based on interim PET results, included: standard consolidation chemotherapy (59.8%) if change in maximum standardized uptake value (ΔSUVmax) of >66% after cycle 2 and >70% after cycle 4 (PET2-/4-), intensive treatment and autologous transplantation (26.8%) if PET2+/4-, and salvage therapy (13.4%) if PET4+ (ΔSUVmax of ≤70%). Among patients with GEP data (n = 107), 38 (35.5%) were PDL1high/PDL2high. Key somatic mutations data (n = 87) included SOCS1 (70.1%), B2M (56.3%), STAT6 (49.4%), TNFAIP3 (47.1%), GNA13 (39.1%), CIITA (37.9%), CD58 (36.8%), and TP53 (29.9%). After a median follow-up of 39.5 months, 2-year progression-free survival (PFS) and overall survival (OS) rates were 86.2% and 93.2%, respectively. In a multivariate model including bulk, aaIPI, and ΔSUVmax PET2/PET4, only bulk and ΔSUVmax PET4 of ≤70% were associated with shorter PFS (hazard ratio, 4.39 [95% confidence interval (CI), 1.28-15.11] and 4.95 [95% CI, 1.71-14.3], respectively), whereas none were associated with OS. The ΔSUVmax-based interim PET4 response emerged as the strongest predictor of patient outcomes in this selected clinical trial population. This trial was registered at www.ClinicalTrials.gov as #NCT01659099.