Background: Disruptions in cancer care adversely affect clinical outcomes. The effect is magnified when treatments are not just protracted, but discontinued prematurely. The incidence of such treatment termination (TT) during radiation therapy has not been well studied. We herein present a single-institution cohort of TT at a large multi-center radiation oncology department over 10 years. Methods: As part of our ongoing departmental quality and safety monitoring program, TTs were prospectively tracked and assessed. A TT was defined as the premature discontinuation of therapy at any point following the initiation of radiation planning. The rate of TT was calculated as a percentage of all new patients who start radiation planning. Each TT between 1/2013 and 1/2023 was analyzed for a root cause, reviewed among multi-disciplinary Quality committee members, and presented monthly to the radiation department. Selected cases sparked discussion and efforts to identify interventions that may decrease the likelihood of similar TTs. Incremental departmental policy and procedure changes to critical pre-treatment evaluation, prospective contouring rounds, on-treatment management, and scheduling processes were implemented, collectively referred to as the “No-Fly” policy. Results: A total of 1,467 TTs were identified and analyzed. 688 (46.9%) involved patients treated with curative intent, 770 (52.5%) with palliative intent, and 9 (0.6%) for benign disease. The most common cause of TT was hospice and/or death (36.5%), 69.1% of which were in patients receiving palliative treatments. Other common causes identified are shown in Table 1. The overall 10-year rate of TT was 5.1% (1,467 of 28,707 planned treatment courses). The rate of TT decreased from 9.3% to 3.3% (2013 and 2022, respectively), with rates decreasing at each iteration of No-Fly from 8.8% (No-Fly 1, 2013-2014) to 5.2% (No-Fly 2, 2015-2018) to 4.0% (No-Fly 3, 2019-2022) (ANOVA p<0.001). Conclusions: Premature termination of radiation therapy can be deleterious for cancer patients. Determining the causes of TTs so that they may be mitigated is integral to effectively managing care and improving safety culture in radiation medicine departments. While some TTs may be unavoidable, open discourse and quality improvement changes are effective in reducing incidents over time. Understanding TTs is complimentary to other nationwide projects exploring how reducing missed radiation therapy appointment rates support access to high quality oncology care.