© 2017 American Society for Radiation Oncology Purpose Radiation therapists play a critical role in ensuring patient safety; however, they are sometimes given insufficient time to perform quality assurance (QA) of a patient's treatment chart and documentation before the start of treatment. In this work, we show the benefits of introducing a formal therapist prestart QA checklist, completed in a quiet space well in advance of treatment, into our workflow. Methods and materials A therapist prestart QA checklist was created by analyzing in-house variance reports and treatment unit delays over 6 months. Therapists were then given dedicated time and workspace to perform their checks within the dosimetry office of our department. The effectiveness of the checklist was quantified by recording the percentage of charts that underwent QA before treatment, the percentage of charts with errors needing intervention, and treatment unit delays during a nearly 2-year period. The frequency and types of errors found by the prestart QA were also recorded. Results Through the use of therapist prestart QA, instances of treatment unit delays were reduced by up to a factor of 9 during the first year of the program. At the outset of this new initiative, nearly 40% of charts had errors requiring intervention, with the majority being scheduling related. With upstream workflow changes and automation, this was reduced over the period of a year to about 10%. Conclusions The number of treatment unit delays was dramatically reduced by using a formal therapist prestart QA checklist completed well in advance of treatment. The data collected via the checklist continue to be used for further quality improvement efforts.