Analysis of monotherapy versus combination interventions under investigation in phase 1 oncology drug trials that allowed patients <18 years from September 27, 2007 to May 1, 2019, with year defined by trial start date on ClinicalTrials.gov. A, Proportion of drug trials evaluating combination of multiple drugs (combo) or single agents (mono) per year; n = 599. B, Treatment modalities as a function of trial ages of eligibility for enrollment; n = 599. Pediatric only: maximum age ≤ 18 years; child, adolescent and young adult (AYA): maximum age ≤ 21 years; pediatric and adult: lower age range < 18 years, maximum age > 21 years. C, Combination drug treatment modalities per year from September 27, 2007 to May 1, 2019; n = 342