Radt-27. bone marrow suppression after reirradiation therapy with proton versus photon therapy for recurrent glioma

Abstract INTRODUCTION High grade radiation-induced lymphopenia has been implicated to worse prognostic outcomes and overall survival in numerous malignancies. Understanding factors which influence lymphopenia can help optimize radiation delivery and overall patient care. Proton therapy (PRT), due to its distinct dose fall-off, spares tissues and organ-contained blood pools from excess radiation compared to conventional photon radiation (XRT) and may thereby reduce hematologic toxicity. METHODS Patients with confirmed glioma diagnosis who underwent reirradiation with either PRT or XRT were retrospectively analyzed. Complete blood counts (CBCs) within one month of reirradiation start, and CBCs within one month of reirradiation completion were reviewed. Values of interest included total white blood cell (WBC), absolute neutrophil count (ANC), and absolute lymphocyte count (ALC). Dosimetric data was collected to assess whole brain mean dose (WBMD), and volume of brain tissue receiving 5-50 Gy (V5-V50). RESULTS 16 patients receiving either PRT (n = 8) or XRT (n = 8) were included. Treatment doses ranged from 35-59.4 Gy. Across all patients there was a decrease in WBC, ANC, and ALC after reirradiation in both PRT and XRT (p < 0.05). Compared to reirradiation with XRT, patients who underwent PRT had significantly lesser reductions in WBC (PRT: 8.60% decrease, XRT: 22.58% decrease; p < 0.05). PRT reirradiation also resulted in lesser ANC and ALC reductions, though did not reach statistical significance. On dosimetric comparison of XRT and PRT plans, PRT demonstrated significant reductions in V5, V10, V20, and V30 (p < 0.05). V5 through V40 and WBMD were significantly correlated with decreased WBC, ANC, and ALC (p < 0.01). CONCLUSION Preliminary results show PRT reirradiation leads to decreased WBMD and volumes of brain tissues receiving 5 to 30 Gy. Given that these parameters are associated with greater reductions in lymphocytic counts, PRT reirradiation may confer reduced hematologic toxicity in patients seeking adjuvant radiation therapy.