European Prospective Investigation Into Childhood Cancer

Inclusion Criteria: Age: ALL and favorable biology brain tumors: Patient must be between 3 years and 21 years of age at the time of consent. Diagnosis: * ALL: Patients must have newly diagnosed B-cell ALL, T-cell ALL, or mixed phenotype acute leukemia confirmed by immunophenotyping by flow cytometry. * Brain tumors: Patients must have newly diagnosed favorable biology brain tumor confirmed by either pathology report, imaging and/or biochemical studies including low-grade gliomas, medulloblastoma, ependymoma, pituitary tumors, germ cell tumors. Treatment: * ALL and brain tumors: Participants may be children on a clinical trial or "as per" a clinical trial. * ALL: Patients will receive standard leukemia treatment. * Brain tumors: Patients will be treated depending upon standard approach with surgery, chemotherapy, radiation therapy. Patients receiving autologous stem cell transplantation as part of their regimen may be included. Location: ALL and brain tumors: Patients must be receiving treatment at one of the participating centers. Timing: * ALL: Parents/guardians must consent and patient assent, if applicable, within three working days of diagnosis. * Brain tumors: Parents/guardians must consent and assent, if applicable, prior to initiation of chemotherapy, radiation or stem cell transplant. Exclusion Criteria: ALL and brain tumors: * Patients with relapsed or progressive disease, exclusive of patients with unresectable low-grade gliomas who have progressive disease. * Patients with history of other primary malignancy. * Patients with other medical conditions not associated with the malignancy that may interfere with nutritional status/growth and/or microbiome composition, like patients with Down's syndrome, metabolic disorders or celiac disease. Patients with genetic predisposition that may interfere with nutritional status/growth. ALL: * Patient plans to receive hematopoietic stem cell transplant. * Mixed lineage leukemias who receive AML-based protocols. Brain tumors: Children/adolescents who will be managed by observation or surgery only, or diagnosed with ATRT or high-grade gliomas.