Distribution of LC50 values as a measure of in vitro resistance to PRD within potentially clinically poor PRD responders (blasts at day 0 ≥1,000/μL of peripheral blood) in clinical good (blasts day 8 of <1,000/μL, n = 114) and clinical poor (blasts day 8 of ≥1,000/μL, n = 12) responders to a 1-week systemic PRD monotherapy (plus one intrathecal injection with methotrexate at day 1) in childhood ALL.