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High-dose chemotherapy with autologous stem cell transplantation (ASCT) is a key consolidative therapy for primary central nervous system lymphoma (PCNSL). However, the optimal regimen for mobilizing haematopoietic stem cells remains undefined in this population, leading to unpredictable failure rates and suboptimal cell yields. We retrospectively analysed data from 142 patients with histologically confirmed PCNSL who underwent stem-cell mobilisation before ASCT. Three mobilisation strategies were compared: high-dose cytarabine (HD-Ara-C)-based mobilisation (n = 64), plerixafor plus G-CSF (n = 60), and cyclophosphamide (CTX, n = 18). Mobilisation success, CD34⁺ cell yield, toxicity, and predictors of collection outcomes were evaluated. Patients in the HD-Ara-C-based mobilisation group achieved a 100% success rate, significantly outperforming CTX (77.8%) and steady-state mobilisation (98.3%) while delivering a markedly superior median CD34⁺ cell yield (20 × 10⁶/kg vs. 12.7 and 10 × 10⁶/kg, p < 0.001). Notably, 79.7% of patients in the HD-Ara-C-based mobilisation group were classified as "very good" mobilisers,with all "poor" mobilisers confined to the other groups. Crucially, we identified and validated the first-day peripheral blood CD34⁺ cell count as a powerful predictor of collection outcome. A threshold of > 31 cells/µL predicted successful collection (> 2 × 10⁶/kg) with an AUC value of 0.94 (98.6% specificity, 78.6% sensitivity). Higher thresholds predicted optimal and high-yield collections. Although grade 4 thrombocytopenia was more common with HD-Ara-C, it was manageable with supportive care. HD-Ara-C-based mobilisation is a highly effective strategy for PCNSL, ensuring universal success and maximizing CD34⁺ cell yields. The first-day CD34⁺ cell count provides a robust, real-time tool for guiding apheresis. HD-Ara-C-based mobilisation should be considered the preferred regimen for fit patients, with steady-state mobilisation as an alternative for selected cases.