OBJECTIVES: To examine the trends in radiation therapy (RT) utilization by a population based breast cancer cohort in Ontario, Canada.  METHODS: The provincial cancer registry maintains cancer specific databases and provided a breast cancer cohort based on diagnosis dates from April 1, 2005 to March 31, 2010. Staging information was also available. The cohort was then linked, by their encrypted health card number, to linkable administrative datasets that are maintained by the Institute for Clinical Evaluative Sciences (ICES) such as RT utilization.  RESULTS: An all female breast cancer cohort (N=39,656) was identified over a five-year timeframe and the average age was 61.6 ± 14.0 years.  Approximately, two thirds (N=25,225) of patients received RT and staging information was available for 22,988 patients (stage I = 9,541; stage II = 8,516; stage III = 4,050; and stage IV = 881). Patients had an average of 1.4 ± 0.7 (stage I) number of RT courses, 1.8 ± 1.1 (stage II), 2.5 ± 1.3 (stage III), and 2.8 ± 2.4 (stage IV).  The percent ratio of conventional RT to intensity modulated RT (IMRT) was 70.9%:16.6% (stage I), 71.6%:11.3% (stage II), 74.6%:4.6% (stage III), and 72.7%:12.6% (stage IV). For the non-IMRT cohort with a primary cancer (N=30,887), the average number of fractions per course was 18.1 ± 9.2.  CONCLUSIONS: From 2005 – 2010, almost two thirds of a Canadian female breast cancer cohort received RT and the average number of courses increased with stage. A similar trend was observed with the type of RT (coventional RT utilization increased with stage) but peaked at stage III and decreased at stage IV, likely due to palliation. The next step is to apply unit costs to the number of fractions per subgroup and to also obtain RT-planning and radiation therapist times.