Increased Number of Beam Angles Is Associated With Higher Cardiac Dose in Adjuvant Fixed Gantry Intensity Modulated Radiation Therapy of Left-Sided Breast Cancer

Purpose: To analyze the relationship between angle number and mean heart dose (MHD) in adjuvant fixed gantry intensity modulated radiation therapy (FG-IMRT) treatment of left-sided breast cancer as is currently practiced in the community.

Methods And Materials: We performed a retrospective, multi-institutional review of women with left-sided breast cancer receiving adjuvant FG-IMRT between 2012 and 2014, encompassing 85 centers in 15 states. Bivariate and multivariate regression analyses were done to identify factors associated with MHD. Long-term cardiac risk was estimated according to a previously published model.

Results: Of the 538 women included, 284 had >2 gantry angle treatment plans (multi-angle), and 254 had 2 gantry angle (standard) plans. Median MHD was higher in patients with multi-angle plans compared with standard (median 475 vs 203 cGy). Number of gantry angles was significantly associated with MHD, with multi-angle plans independently increasing MHD by 229 cGy. Absolute risk of acute coronary events 20 years after treatment was estimated as 7 excess events per 1000 women for standard plans, compared with 12 excess events for multi-angle plans.

Conclusions: Fixed gantry IMRT breast treatment plans with >2 gantry angles were associated with increased MHD, which translated to an increased cardiac risk. Clinicians should account for this potential drawback in treatment technique when assessing overall plan quality.