ImportancePlasma measurements of RB1 circulating tumor DNA (ctDNA) after completion of treatment may be associated with the development of metastases in patients with retinoblastoma.ObjectiveTo determine if the absence of previously detectable plasma ctDNA is associated with metastasis-free survival in patients with a minimum of 1 year follow-up after treatment of retinoblastoma.Design, Setting, and ParticipantsThis cohort study was conducted from June 2019 to September 2023. Patients with retinoblastoma who had measurable ctDNA levels at diagnosis and had repeated ctDNA measurements after ocular treatment (enucleation or intra-arterial chemotherapy) with a minimum of 1 year of follow-up (mean [SD], 28.2 [10.3] months) were included in the study. Patients were recruited from a single-center, tertiary cancer hospital.ExposureMemorial Sloan Kettering’s New York State–approved gene test, which interrogates 129 known cancer genes (called ACCESS), was performed on plasma samples before and after ocular treatments. All exons of the RB1 gene are included in the test and listed as ctDNA in this article.Main Outcomes and MeasuresPlasma ctDNA level before treatment, after completion of ocular treatment, and development or absence of metastases.ResultsA total of 24 patients (mean [SD] age, 20.7 [17.1] months; 15 female [62.5%]) were included in the study. None of the 23 patients who had a measurable ctDNA level and then no detectable ctDNA level after completion of ocular treatment developed metastases with a minimum of 1 year of follow-up. One patient had persistent measurable ctDNA after initial treatment and developed metastases.Conclusion and RelevancePatients with retinoblastoma who had a measurable ctDNA level at diagnosis did not develop metastases if the plasma ctDNA level became unrecordable after ocular treatment; 1 patient who had persistent measurable ctDNA after treatment did develop metastasis.