PURPOSE: Many patients face barriers to cancer care, leading to disparities in cancer-specific outcomes. Specifically, missing multiple radiation treatments can be associated with increased locoregional recurrence. We sought to assess the frequency and reasons for missed radiotherapy among accredited cancer programs participating in a national quality improvement (QI) collaborative addressing barriers to care. METHODS: The Breaking Barriers National QI Collaborative conducted through the American College of Surgeons Cancer Programs enrolled 342 accredited cancer programs, prospectively recording patients scheduled for a 15-45-day course of radiotherapy over five separate, 60-day data collection periods (March-December 2023). Programs identified those who missed ≥three treatments, referred to as at risk of worse outcomes, and queried reasons for missed radiotherapy. Kruskal-Wallis tests assessed differences in rates of missed treatments. RESULTS: In total, 332 programs (97.1%) identified at-risk patients, totaling 5,221 patients who missed ≥three treatments. The median percentage of at-risk patients per program was 9.4% (IQR, 4.5-16.5). Programs located in the Northeast (median, 11.3% [IQR, 5.4-17.3]) had the highest proportion of at-risk patients ( CONCLUSION: Barriers to completing radiotherapy are prevalent among accredited cancer programs nationwide. Future work developing interventions in response to identified barriers in this national QI collaborative may be scalable to other areas of cancer care to improve outcomes.