Flow cytometric detection of leukemic blasts in cerebrospinal fluid predicts risk of relapse in childhood acute lymphoblastic leukemia: a Nordic Society of Pediatric Hematology and Oncology study - Thastrup, M; Marquart, H V; Levinsen, M; Nordic Society of Pediatric Hematology and Oncology (NOPHO) (Leukemia, 2020)