Larotrectinib in Treating Patients With Previously Untreated TRK Fusion Solid Tumors and TRK Fusion Relapsed Acute Leukemia

Inclusion Criteria: * Patients must be =\< 30 years of age at the time of study entry * COHORT A: Patients must have a histologic diagnosis of infantile fibrosarcoma with an NTRK1, NTRK2, or NTRK3 fusion identified in a Clinical Laboratory Improvement Act/College of American Pathologists (CLIA/CAP) certified laboratory. Fusions may be identified by fluorescence in situ hybridization (FISH) or molecular techniques (reverse transcriptase-polymerase chain reaction \[RT-PCR\] using primers flanking the fusion junction or next generation sequencing). For fusions identified by FISH, an ETV6 rearrangement is sufficient for eligibility in Cohort A. Identification of the upstream TRK fusion partner is not required. * COHORT B: Patients must have a histologic diagnosis of any solid tumor other than infantile fibrosarcoma, including central nervous system (CNS) tumors but excluding high grade gliomas. An NTRK1, NTRK2, or NTRK3 fusion must be identified in a CLIA/CAP certified laboratory. Fusions may be identified by FISH or molecular techniques (RT-PCR using primers flanking the fusion junction or next generation sequencing). For fusions identified by FISH, there must be an identified rearrangement in NTRK1, NTRK2, or NTRK3 (e.g., an ETV6 rearrangement is not sufficient for eligibility) unless the patient has a diagnosis of congenital mesoblastic nephroma in which case an ETV6 rearrangement is sufficient for eligibility. Identification of the upstream TRK fusion partner is not required…