Improvement in Cardiac Dosimetry by Optimizing Heart-Sparing Breast Radiotherapy

Abstract

Darby et al. demonstrated that cardiac events after breast radiation therapy (RT) are associated with mean heart dose (MHD). MHD is dependent on treatment fields and can be reduced by various techniques, such as deep inspiratory breath hold (DIBH), prone positioning, and cardiac blocks. The purpose of this study was to assess changes in MHD with advancements in modern RT by technique and treatment fields. This retrospective study included breast cancer patients treated from 2010 to 2018 at a single institution in an IRB approved registry. MHD was analyzed with respect to clinical variables, including laterality, cardiac sparing technique, and treatment fields, including regional nodal irradiation (RNI) with or without internal mammary nodal irradiation (IMNI). 765 patients (431 left, 315 right, 19 bilateral) were included. The average MHD was 1.87 Gy (SD 1.36 Gy, median 1.43 Gy) for left sided cases vs. 0.71 Gy (SD 0.80 Gy, median 0.51 Gy) for right sided cases (p<0.0001). Of the 43 patients that had a MHD > 4 Gy, bilateral cases were at a highest risk (21.1%), vs. left sided (7.7%) and right sided cases (1.9%) (p<0.0001). For left sided cases, average MHD without RNI, RNI without IMNI, and RNI with IMNI were 1.40 Gy (SD 0.98 Gy, median 1.16 Gy), 2.01 Gy (SD 1.15 Gy, median 1.63 Gy), and 3.48 Gy (SD 2.17 Gy, median 3.21 Gy), respectively. Following the 2013 publication of Darby et al, the average MHD significantly decreased for left sided RT cases (2.02 vs 1.82 Gy, p= 0.039) due to institutional adoption of cardiac-sparing techniques. For left sided cases w/o RNI, use of cardiac sparing techniques reduced average MHD from 1.61 Gy (SD 1.09 Gy, median 1.36 Gy) to 1.22 Gy (SD 0.99 Gy, median 0.99 Gy) with DIBH and 1.02 Gy (SD 0.56 Gy, median 0.78 Gy) with prone positioning respectively (p<0.0001), with no difference between techniques (p = 0.42). For left sided cases with RNI, DIBH reduced average MHD from 2.4 Gy (SD 0.13 Gy, median 2.02 Gy) down to 2.1 Gy (SD 1.57 Gy, median 1.6 Gy), p= 0.046. For left sided cases of RNI with IMNI, average MHD was 3.09 Gy (SD 2.38 Gy, median 2.57 Gy) with partially wide tangents compared to 3.99 Gy (SD 1.82 Gy, median 3.79 Gy) with matched electron fields (p= 0.053). For left sided cases with IMNI, the MHD was 3.48 Gy (SD 2.31 Gy, median 3.21 Gy) with DIBH compared to 3.64 Gy ( SD 2.16 Gy, median 3.37 Gy) without DIBH (p=0.27). The MHD for left sided RT with reconstruction was 1.94 Gy (SD 1.21 Gy, median 1.53 Gy) and without reconstruction was 1.85 Gy (SD 1.42 Gy, median 1.39 Gy). Cardiac doses were lower following the publication of Darby et al. Overall the average MHD was <1 Gy and <2 Gy for right and left breast cancers, respectively, by increased utilization of cardiac sparing techniques such as DIBH and prone positioning.