Drivers of Variation in the Rate of Radiotherapy Following Lumpectomy in the Military Health System.

Introduction: Radiation therapy (RT) following lumpectomy has been shown to improve survival. However, little is known about drivers of RT within the Military Health System (MHS). We examined how factors previously shown to affect RT rates after lumpectomy in the civilian population and factors exclusive to service members such as rank, service branch, and MHS catchment area are associated with undergoing RT within 6 months after lumpectomy. Materials and methods: We conducted a retrospective cohort study using the 2007-2019 MHS Data Repository (MDR) to obtain data from 9681 service members or dependents who had lumpectomy to treat breast cancer and remained in the system at least 6 months after their lumpectomy. We used Cox-Proportional Hazards to model time until their first RT session and examined how demographic and service-related factors affected rates of RT following lumpectomy. Results: Within the sample, 57.4% of lumpectomy patients received RT within 6 months after surgery. Receiving timely RT was found to be positively associated with older age (Cox Model Benefit Ratio 95% CI [1.01, 1.02]), senior rank (Benefit Ratio 95% CI [1.00, 1.31]), and having a Mental Health diagnosis (Benefit Ratio 95% CI [1.40, 1.60]) when controlling for all other variables in the model. Using catchment area RT rates from 2007 to 2017 and modelling data from 2018 to 2019, it was found that timely RT is positively associated with being in the catchment areas at the top quartile of historical RT rates (Benefit Ratio 95% CI [1.06, 1.47]). Conclusion: We identify important variation in receiving RT after lumpectomy both when comparing MHS rates to known rates in the general population and when comparing rates across catchment areas where military personnel are stationed. Lumpectomy patients in the MHS who are stationed in catchment areas with higher historical RT rates are themselves more likely to undergo RT.