The goal of this observational study is to learn about, the systemic exposure to dexamethason in childhood acute lymphatic leukemia (ALL). The main questions it aims to answer are: * How does the intake of dexamethasone correlate with systemic exposure to dexamethason? * 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 done as part of treatment, a sample of cerebrospinal fluid will also be taken to analyze dexamethasone. * Visit the clinic four times for a DXA scans to measure bone density and 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. Additionally, participants will fill out questionnaires to track their daily physical activity levels.