Prophylactic Cranial Irradiation prior to HCT for Acute Lymphoblastic Leukemia: To Boost or Not To Boost

Background Total body irradiation (TBI) with or without cranial radiation boost (CRB) is an integral component of conditioning prior to allogeneic hematopoietic cell transplantation (allo-HCT) in acute lymphoblastic leukemia (ALL). The benefit of CRB is not yet established. Methods This is a retrospective single center cohort study. Between January of 2003 and April of 2019, electronic medical records of 166 patients with ALL were retrospectively reviewed. One hundred forty-three patients with ALL and no prior central nervous system (CNS) involvement were included. Patients were divided into two cohorts according to cranial radiation boost (cohort-1: CNS-/CRB+ (110/143, 77%) and cohort-2: CNS-/CRB- (n=33/143; 23%). No patients received post-transplant prophylactic intrathecal chemotherapy. Results Following alloHCT, 15 patients (10.5%) experienced relapse; 11 relapses (10%) in cohort-1, and 4 (12%) in cohort-2. Four patients (26.6%) experienced systemic medullary relapse with initial central nervous system (CNS) involvement. One patient (6.6%) experienced isolated first central nervous system relapse after allotransplant with no difference between the two cohorts (6.6% vs 0; P-0.59). Age at transplant and phenotypic subtype were predictive of first central nervous system relapse after allotransplant with respective P-values of 0.001 and 0.015. At a median follow-up of 30 months (range: 2.5-128 months), the estimated 3-year overall survival was 61% (95% CI: 53-69), relapse free survival was 60% (95% CI: 52-69) and 3-year central nervous system-relapse-free survival was 99% and 100% in in cohort-1 and cohort-2 respectively, when systemic relapses were censored. There was no statistical significant difference in either survival or relapse free survival between the two cohorts (P > 0.69). Conclusions Our results suggest that augmenting total body irradiation with cranial radiation boost in patients with ALL with no prior CNS involvement did not improve relapse risk in central nervous system or survival outcomes.