Pharmacokinetics of Dexamethasone in Childhood ALL and Reduction in Bone Mineral Density

The goal of this observational study is to learn about systemic and central nervous system (CNS) exposure to dexamethasone in childhood acute lymphoblastic leukaemia (ALL). The main questions it aims to answer are: * How does the intake of dexamethasone correlate with systemic exposure to dexamethasone in blood? * How does systemic exposure to dexamethasone correlate with dexamethasone concentrations in cerebrospinal fluid (CSF)? * Is dexamethasone exposure in blood and CSF associated with clearance of leukemic CNS infiltration? * Does systemic and/or CNS exposure to dexamethasone correlate with neurotoxicity as assessed by questionnaires? * Does systemic exposure to dexamethasone correlate with a reduction in bone mineral density? Participants will: * Continue to receive the best available therapy for ALL in Western Europe. * Have blood samples taken from their central line to measure dexamethasone levels. * When standard lumbar punctures are performed as part of treatment, an additional sample of cerebrospinal fluid will be collected to analyse dexamethasone concentrations and assess leukemic CNS involvement when applicable. * Visit the clinic four times for DXA scans to measure bone density and perform vertebral fracture assessment: within three weeks of starting treatment, six months after starting treatment, one month after finishing treatment, and one year after finishing treatment. Biomarkers related to bone health will also be collected on these days. * Complete validated questionnaires to monitor neurotoxicity and to track daily physical activity levels during treatment.