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Purpose: Analysis of circulating tumor DNA (ctDNA) in the plasma of patients with retinoblastoma and simulating lesions. Design: Retrospective cross-sectional study of the association of plasma ctDNA from retinoblastoma and simulating lesions with disease course. Participants: Fifty-eight Memorial Sloan Kettering Cancer Center patients with retinoblastoma comprising 68 plasma ctDNA samples and 5 with retinoblastoma-simulating lesions. Methods: The ctDNA analyzed with hybridization capture and next-generation sequencing in blood (plasma) of patients who had retinoblastoma or simulating lesions were evaluated for association with clinical course of the disease. Main Outcome Measures: Presence or absence of molecular aberrations in the RB1 gene and correlations with clinical features. Results: RB1 cell-free DNA (cfDNA) was detected in 16 of 19 patients with newly diagnosed, untreated intraocular retinoblastoma and in 3 of 3 patients with newly diagnosed, untreated metastatic disease. It was also present in 3 patients with recurrent intraocular disease before therapy, but was not present in patients with recurrent disease who received intra-arterial chemotherapy, nor in 21 patients who had undergone enucleation for unilateral disease. In 1 patient who had delayed treatment (insurance reasons) and showed rapid growth of the intraocular tumor, the variant allele frequency increased in 1 month from 0.34% to 2.48%. No RB1 mutations were detected in the cfDNA from plasma of patients with simulating lesions (3 with Coats disease and 1 with persistent fetal vasculature [PFV]). In 2 patients, we identified 2 independent RB1 mutations in plasma. Conclusions: Mutations in RB1 were found in the cfDNA from blood of patients with newly diagnosed, untreated retinoblastoma and in patients who showed disease recurrence in the eye after prior treatment, but not in unilateral retinoblastoma after enucleation Levels of ctDNA increase in patients with progressive disease who did not receive any treatment. High plasma cfDNA levels were detected in patients with newly diagnosed metastatic disease, and these levels decreased after systemic chemotherapy was administered. Further validation is needed for measuring the somatic alterations in cfDNA from blood in retinoblastoma that could provide a promising method of monitoring patients in the future.